If an entry in the loop at `_tpl_metaheaders_action` has no content, skip this entry.

I supplemented the existing comments on the methods in which there was not enough information to the standard phpDoc.

All script tags are now surrounded by conditional comments to tell
Internet Explorer 8 and lower to not even bother trying to load the
JavaScript.

This introduces a new dokuwiki\Ui namespace and refactors the Admin
screen into a Ui class. The ultimate goal is to split up the big,
complex functions in inc\html.php in better maintainable classes in the
Ui namespace. This is the first go at it. Others function->class
conversions should follow.

This also switches the icons for our base admin plugins to inline SVG.
(files and styling not included, yet).

We now have two CDNs available. code.jquery.com which is the more
popular one and CDNjs which is the faster one. Plugin authors can use a
plugin hook to easily implement their own preferred CDN. Authors might
even use this event to conditionally load additional JavaScript files.

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.

@scrutinizer-notifier noticed that the $pageid might be false when
page_findnearest looks for it.

This is needed to accomodate for the change in page_findnearest()

This event handles both known core types and unknown types as default

re-add PR #470 again.

This was previously carried out in three separate places. Refactor
that code to use the new function.

Update tpl_pageTitle test to use a manager level admin plugin.

add unit test coverage

    See Issue #1149
    - Show 'admin' or the admin plugin name for admin functions
    - Show action name for user & site actions, e.g. login, sitemape
    - Show page name + action for page actions, e.g. changes, backlinks
    - Show a utf8 pen glyph when editing
    - Otherwise show the page name (as previously)

This reverts commit c2fbf986.

See Issue #1149
- Show 'admin' or the admin plugin name for admin functions
- Show action name for user & site actions, e.g. login, sitemape
- Show page name + action for page actions, e.g. changes, backlinks
- Show a utf8 pen glyph when editing
- Otherwise show the page name (as previously)

This reverts commit 3940c519 as
discussed in #1082.

This reverts commit 362a4f08.

Added placeholder in search textarea

In an older version of PHP a file_exists() call would issue a warning
when the file did not exist. This was fixed in later PHP releases. Since
we require PHP 5.3 now, there's no need to supress any error here
anymore. This might even give a minor performance boost.

As described in the common idioms of the HTML5 spec, mark up navigation as a list inside a `nav` element for better semantics and accessibility.

see:

  * http://www.w3.org/html/wg/drafts/html/master/common-idioms.html#rel-up
  * http://lists.w3.org/Archives/Public/public-html/2013Nov/thread.html#msg6
  * https://dl.dropboxusercontent.com/u/377471/breadcrumb.html

for discussion and background.

many PHPDocs
some unused variables
some dynamically declared variables declared

different types
unused vars
PHPDocs