St theme: use css instance data

Remove files_by_stylesheet hashtable usage while take advantage of the CRStyleSheet's app_data field to save stylesheet related informations such as the parent GFile and whether this is an extension style-sheet.

So reduce the hashtable lookups and use direct access to data. Then simplify memory management using simpler ref/unref'ing.

Last commits is somewhat optional, but makes things a bit cleaner to my eyes (while I've removed the commit 3v1n0/gnome-shell@531267c9 which was somewhat more needed in the previous revision of the MR where I was still using the custom_stylesheets list).