lib/gtk: move default gtk module as a project

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/gtk: move gtk3_6 to gtk::v3_6

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/gtk: move gtk3_8 to gtk::v3_8

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/gtk: fix missing pkgconfig declaration in v3_6 and v3_8

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/gtk: improve documentation on main modules

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
Merge: GTK refactor and fixes

Now uses project and submodules! So one can access different versions using:

    import gtk # default is v3_4
    import gtk::v3_6
    import gtk::v3_8

Pull-Request: #431
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>