DIORITE_GUI_MODE to simulate Unity/GNOME/XFCE

Added environment variable DIORITE_GUI_MODE to simulate Unity/GNOME/XFCE
for debugging purposes.

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Add SlideInRevealer widget

This widget contains arrow button to slide in/hide its child.

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Add function ActionsRegistry.create_action_button

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>