No globals required anymore, somewhat more general approach to sending
data to the test system. Clean access through keys.

Test code for workflow 'show - sectionedit - save/cancel' (with modifications in test classes and 'send_redirect()' and 'act_dispatch()'.

This partly fixes #1883

This makes it more flexible to use on the expense of needing one echo
more.

now comments and line breaks between tags are removed

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).

This introduces an embedSVG() function that can be used at other places.

This fixes saving the actually saved timestamp for deleted revisions in
the change log. Before this, the change log got 0  as timestamp and
therefore used the current time - which might be wrong if the current
second changed in between touching the page and adding the entry to the
changelog.

The event COMMON_WIKIPAGE_SAVE offers the opportunity to save the content by modifying $data['newContent']. However saveWikiText still saves the original $text variable instead of the possibly modified $svdta['newContent'].

Using an HTML entitiy might be problematic in non HTML renderers or when
the output is escaped again.

The header() method of PHP is vulnerable to HTTP Response Splitting
attacks.

This change makes sure the URL passed to send_redirect (and thus to
header()) does not contain any control characters that would be needed
to execute such an attack.

Cleaning input is recommended anyway.

fullpath processing here seems unnecessary, wikiFN($ID) returns a valid filepath for the page text file.

- handle all cases where not an old file exist, or when that old file is
zero size.
- When using old revision normally these files are compressed, use
uncompressed size.

As discussed in #1471, an uninitialized variable will always be
implicitly created when passed to the blank() function. Calling isset()
is thus a no-op. A warning about this behavior has been added to the
function comment.

This makes the saveWikiText() function a little easier to read and moves
external edit handling to its own function. Behavior stays the same
(tests are unchanged).

In addition a new event COMMON_WIKIPAGE_SAVE is introduced that makes
intercepting and acting on page saves much easier than possible before.

Developers can:

* prevent saves by either preventing the default action or overwriting
  the contentChanged field in a BEFORE hook
* enforce saves even when no content changed by overwriting the
  contentChanged field in a BEFORE hook
* Adjust the saved content by modifying the newContent field in a BEFORE
  hook
* Adjust the stored change log information (summary, type, extras) in an
  AFTER hook
* Easily know if a page was deleted, created or edited by inspecting the
  changeType field
* what ever they want before or after a wiki page is saved

This allows to quickly check if a variable is empty but keeps strings
and numbers evaluating to zero.

Implements
https://bugs.dokuwiki.org/index.php?do=details&task_id=2525

We no longer support IE6, so hashes in URLs no longer need to be
protected when redirecting.

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.

many PHPDocs
some unused variables
some dynamically declared variables declared

Restores vanished changes of #84