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>