GtkFileChooser: Add a menuitem for 'sort directories first'

This has been requested repeatedly.
GtkIconTheme: Improve an error message

Mention the name of the theme when an icon lookup fails.
https://bugzilla.gnome.org/show_bug.cgi?id=687963
Only warn once if inhibit-related dbus calls fail

No need to spam about it.

https://bugzilla.gnome.org/show_bug.cgi?id=734140