Support CSD windows

This patch adds support for the titlebar slot in GtkWindow.
The rendering of windows with csd titlebar is somewhat broken
in glade.

https://bugzilla.gnome.org/show_bug.cgi?id=700914
GladeWindowEditor: Added custom control for client side decorations

This is needed so that when the command is issued, it can also
include the child deletion as a part of the command group.

When a window is using CSD, mark the title, decoration and
hide-when-maximized editors as insensitive, since they affect
non-CSD decorations.
Use proper mechanisms to deal with dependent properties

Make desensitizing of title, decorated and hide-titlebar-when-maximized
part of the undo history, and also reset them to their default values.
glade-gtk-window.c: Reuse translatable string

Avoid the possibility of translatable string redundancy.
Depend on GTK+ 3.15

Avoid GTK_CHECK_VERSION ifdefs working around the absense of
gtk_window_get_titlebar().