Adds missing and sorts existing language items

Also:

* Improves some language items.
* Moves `wcf.acp.user.rank` language items into `wcf.acp.user` since forth-level language categories aren't supported.
* Adds missing (i.e. currently untranslated) English language items, but they are commented out, thus not installed and still "appear to be missing". If you want to translate them, you can simply search for `<!-- todo: ` in `en.xml`.
Does some notification-related code/documentation cleanup
Removes code that became obsolete with DBO::getObjectID()
Merge pull request #142 from Gravatronics/enhancement/cleanup

Does some notification-related code/documentation cleanup
Fixes DashboardBoxAction::updatePosition() if no active box exists
Fixes activity point overlay
Replaces capture with assign
Removes obsolete checks
Merge branch 'master' of github.com:WoltLab/com.woltlab.wcf.user into enhancement/getObjectID
Merge pull request #144 from Gravatronics/enhancement/getObjectID

Removes code that became obsolete with DBO::getObjectID()
Reverts changes in en.xml
Merge branch 'master' of github.com:WoltLab/com.woltlab.wcf.user into enhancement/languageItems
Merge pull request #135 from Gravatronics/enhancement/languageItems

Adds missing and sorts existing language items
Fixes CSS classes names of content dashboard box list
Fixes dashboard sorting

Previously, if one box type was sorted, the boxes of the other type where disabled since the fetched boxes (`$this->boxes`) were of both types.