listbox: Implement gtk_list_box_insert()

... to make it possible to insert rows in the middle of the list without having
to fiddle with the sort functions. One of the first users is going to be Glade.

https://bugzilla.gnome.org/show_bug.cgi?id=705558
listbox: Reorder code

This just moves the gtk_list_box_insert function to where the rest of the
public API is defined.

https://bugzilla.gnome.org/show_bug.cgi?id=705558
Updated Spanish translation
dnd: Motif DnD is no longer supported, update docs.
Reset cursor when mouse leaves a toplevel window.

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=692548