cursor: Request the OSK when the cursor is disabled

Register 'org.gnome.SettingsDaemon.Cursor' on the session bus. Add a
D-Bus property 'ShowOSK' to indicate whether the on-screen keyboard
should be enabled, which is triggered when a direct touch device is used
for input.

https://bugzilla.gnome.org/show_bug.cgi?id=692772