typo

This adds some simple defaults for the guaranteed placeholders. This
ensures plugins can safely use these placeholders even when a template
did not implement them.

This doesn't really do much except wrapping the functions into a
namespaced class. Autoloading takes care of loading the file.
It's now possible to call lib/exe/ajax.php multiple times in a test
request.

added test coverage

While DokuWiki suppresses PHP Notices they are still a code smell and
should be fixed. This fixes some PHP Notices that occurred.

Some of these fixes could be refactored into nicer code once we move to
PHP 7 and get access to the `??` operator.

No need to check those seperately. The correct order should be ensured
already.

This also removes duplicated code from ajax.php

This is important because the $cache->cache would have been set already and have no impact on the delivered result. Meaning: two different requests could have resulted in the same result.

Remove the flavour feature. It was some kind of a hassle. If you really want to filter out styles, you have to write a plugin either way and implement the tpl metaheaders event as well and handle everything your self.

The CSS modification is more like the JS_SCRIPT_LIST now. It adds some overhead, but not too much. It still gives great flexibility over the CSS.

Changed the cache initialisation: it does not use the list of files with an md5 (due to redundancy and time consumption) but the flavour and the type (previously removed).

Use the flavour system to create individual css files. Only the default flavour (style / style.ini) will have the default styles for interwiki and filetypes. There are two new events:

 1. CSS_STYLES_INCLUDED to modify the list of stylesheet files for a certain mediatype or prevent the mediatype or styles within it. This event is fired for the interwiki and filetypes too, to prevent that they are included
    * The Data that is being sent to the event contains: list of files or the keyword DW_DEFAULT for the interwiki / filetypes; the mediatype, the flavour, if the data should be encapsulated by a default @media and the encapsualation prefix (e.g. @media screen)
 2. CSS_CACHE_USE to prevent the use of a cached version of the CSS, just like the JS event that already exists.

Add flavours to template ini files. The template will now be checked for multiple ini files and inserts them as a custom flavours. The css.php can detect these flavours and reads the stylesheet + replacements from there. If the flavour is not the default one (style.ini), replacements are loaded from the default flavour first to ensure that the colours are available. Flavours can define their own set of replacements.

Remove type and caching mechanism. The cache will be checked later and the type will be replaced by an md5 hash of the list of style files.

Add Event to modify the list of javascript files before they are processed. This allows plugins to not have their - or other plugins - script delivered to the client. Creating the list of files upfront might add a little overhead which I think is OK when you can be certain that only JavaScript that really is needed will be delivered to the client.

Multiple Javascript requests, e.g. to only send the jQuery part or just editor/administrator scripts, could be implemented with this modification (I know, jQuery has already been split).

jQuery (and UI and Migrate) are now loaded separately from the rest of
the JavaScript. This adds at least one HTTP request more but has some
advantages:

* browsers can cache it independently
* the cache is only invalidated when versions update
* we do not apply any transformations (replacements, minimizing, etc) on
  this code anymore which makes our dispatcher faster for the other JS
* browsers seem to load (not execut) both (jquery and other) parallel,
  which might increase download speed a bit

This split allowed for the introduction of a new config: jquerycdn. When
enabled the 3 jquery files are loaded from jQueries CDN. This adds
another two HTTP requests but:

* since it's another host those files do not apply to the 4 request per
  host limit and can be loaded (not executed) in paralell which might
  increase download speeds a bit
* the CDN is distributed worldwide which means files are requested from
  the closest location, increasing the download speeds
* since these files/CDN are very popular, chances are high that people
  already have them cached in their browsers, reducing the download time
  to 0 and effectiely halving the javascript needed to download

The option currently defaults to 'off', but I would argue 'on' would be
the better default.

In php7 constructors with the same name as the class are deprecated. __construct() should be used.

Depending on the server-configuration this could result in a warning which would break the API.

Prevents overwriting already added stuff.

#1327

restoring of mediafiles failed