device/xi2: Implement get_core_device()
device/xi2: Implement get_core_device()

We ask XI2 to get the client pointer for CLUTTER_POINTER_DEVICE, and
we use the attached keyboard device for CLUTTER_KEYBOARD_DEVICE. For
everything else, we return NULL.

jigpu/clutter,Distrotech/clutter,jigpu/clutter,spatulasnout/clutter,djdeath/clutter-android,djdeath/clutter-multithreaded,Distrotech/clutter,collects/clutter,djdeath/clutter,spatulasnout/clutter,dlespiau/clutter,GNOME/clutter,ebassi/clutter,jigpu/clutter,ebassi/clutter,nobled/clutter,jigpu/clutter,djdeath/clutter-multithreaded,spatulasnout/clutter,nobled/clutter,kerrickstaley/clutter-vala,djdeath/clutter,collects/clutter,heysion/clutter-clone,heysion/clutter-clone,djdeath/clutter,jigpu/clutter,Distrotech/clutter,Distrotech/clutter,spatulasnout/clutter,djdeath/clutter-multithreaded,Distrotech/clutter,djdeath/clutter-android,kerrickstaley/clutter-vala,kerrickstaley/clutter-vala,djdeath/clutter-android,djdeath/clutter,ebassi/clutter,kerrickstaley/clutter-vala,ebassi/clutter,djdeath/clutter-multithreaded,jigpu/clutter,heysion/clutter-clone,heysion/clutter-clone,djdeath/clutter,GNOME/clutter,Distrotech/clutter,collects/clutter,nobled/clutter,GNOME/clutter,collects/clutter,ebassi/clutter,djdeath/clutter,dlespiau/clutter,djdeath/clutter-android,GNOME/clutter,dlespiau/clutter,collects/clutter,djdeath/clutter-android,heysion/clutter-clone,spatulasnout/clutter,GNOME/clutter