[ticket/12393] Allow users to quickedit their posts

This only has a basic set of features right now. It does not include any
posting buttons. Triggering quickedit, cancelling quickedit, submitting the
edited post, and switching to the full editor works in general.

Previous issues that have been fixed:
* Deleting polls on quickedit
* Edit button opens up more quickedit windows below each other
* Possibility to open up more than one quickedit window (solution might need to be improved)

Ticket: https://tracker.phpbb.com/browse/PHPBB3-12393