Use %Y%m%d%H%M in version from git

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Add class PRIMARY_TOOLBAR to header_bar_revealer

It may make it look better in Unity

Signed-off-by: Jiří Janoušek <janousek.jiri@gmail.com>
Add ActionsRegistry.activate_action() helper

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