Added Observer event view_backend_layout_head.

Allows people to insert code in the HEAD section of the backend.
little formatting and cleanup of Page::findBySlug
Merge pull request #511 from andrewmman/patch/findBySlug

little cleanup and some formatting of Page::findBySlug
Merge pull request #509 from wolfcms/headevent

Added Observer event view_backend_layout_head.
Fixes the curious case of the missing sort order for FileManager