kconfig: gconf: migrate to GTK 3

This commit switches from GTK 2.x to GTK 3, applying the following
necessary changes:

 - Do not include individual headers
 - GtkObject is gone
 - Convert Glade to GtkBuilder

Link: https://docs.gtk.org/gtk3/migrating-2to3.html


Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>