base-item: Make trash a virtual method and implement it for local items

One can use can_trash to find out whether a PhotosBaseItem sub-class
implements it or not. As a positive side-effect, can_trash is no longer
TRUE for remote collections.

https://bugzilla.gnome.org/show_bug.cgi?id=720222
Add PhotosDeleteNotification

https://bugzilla.gnome.org/show_bug.cgi?id=720222
selection-controller: Add missing whitespace
view-container: Remove redundant NULL check
view-container: Plug a memory leak