lib/gtk: add `GtkBox` to containers hierarchy

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/gtk: add `GtkWidget::destroy|show|hide` and avoid conflicts

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
lib/gtk: fix import from `GtkEntry::text`

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
Merge: Minor Modifications to GTK3

Pull-Request: #1068
Reviewed-by: Jean Privat <jean@pryen.org>