ArticleExtradata class and other changes to models.php

* ArticleExtradata class added. This class gives plugins the possibility
  to assign additional data to articles.
* Abstract class KVStorage added. ArticleExtradata and PluginKVStorage
  extend from it.
* Added dbversion function, it returns the version of the database model
  currently in use. The version is stored in the PREFIX_meta table.
  If there is no PREFIX_meta table, dbversion returns version 0.
  This will be useful for updating the system.
* New MySQL tables.
Added silent mode to abstract KVStorage class.

Activated silent mode will return NULL instead of throwing an
DoesNotExistError Exception, if a key does not exist.
Plugins can now hook into the Article editor.

* Plugins can display additional fields in the article editor.
* Plugins can perform actions when saving an article and even veto the
  saving.
* Plugins can get their ArticleExtradata objects.
Merge branch 'feature-plugin-articledata' into develop
Included jQuery to the libraries.
Sidebar blocks of Article editor can be expanded/collapsed.
Made collapse/expand feature more obvious.

* The underlining on hovering makes it more obvious that something will
  happen, if you click on it.
* Also cleaned up the stylesheet a bit.
ste:comment_form can now generate a previewbtn by default.
ste:default_delete_yesno introduces

This tag makes it a bit easier to write backend templates. It
automatically generates the typical  dropdown + button combo for
mass-deleting entries.
Better(?) tag editor.

* Look&Feel of the Tag editor now matches better with the rest of the
  backend.
* Deleted templates, functions and translations that are not longer
  needed.
Better .listtab style

* Text inputs in the cells of a table.listtab are now extended to the
  full cell width.
* Explicit padding of cells.
Awesome background / watermark for backend.

Many pages had a lot of empty white space, that watermark fills this space
without being too annoying.
Added missing comma in translations/de.php
Merge branch 'feature-ui-improvements' into develop
Implemented on_* tags. UNTESTED!

on_article / on_tag / on_section will execute and return its content, if
currently a(n) article / tag / section is requested.
articles_get can take section/tag as a var now.

* For better cooperation with the on_* tags (which are tested now).
* Should also reduce the number of SQL-Queries if used correctly and
  therefore improve performance.
Default template now uses new features.
Removed debug stuff from standard.html template.
Merge branch 'feature-easier-templating' into develop
Article::test_urlname implemented.