findbar: port to GtkButton

* GdHeaderSimpleButton -> GtkButton

https://bugzilla.gnome.org/show_bug.cgi?id=705881

Signed-off-by: Yosef Or Boczko <yoseforb@gmail.com>
window: port to GtkHeaderBar, GtkButton and GtkRevealer

* GdHeader*Button -> GtkButton subclass
* GdHeaderButton -> GtkHeaderButton
* GdRevealer -> GtkRevealer

https://bugzilla.gnome.org/show_bug.cgi?id=705881

Signed-off-by: Yosef Or Boczko <yoseforb@gmail.com>
Remove libgd submodule

https://bugzilla.gnome.org/show_bug.cgi?id=705881

Signed-off-by: Yosef Or Boczko <yoseforb@gmail.com>