Deleting a gallery also removes relations

All gallery<->image relationships are now also deleted from
RELATIONS_TABLE
Gallery edits can be saved now

Simple ajax call. Frontend also changes immediatly.
Added GalleryBackendController Class

So far, it ha a static method to get all image-type posts
Refactored Admin JS modules

On top of effectless refactoring, thew NewGalleryWidget now closes after
a gallery has been inserted and new galleries can now be editet without
reloading the page.