In order to separate pointer and keyboard grabs, we need to
introduce a keyboard grab interface but first we must rename
some generic types to denote which device is holding the grab.

Type renames:

wl_grab_interface -> wl_pointer_grab_interface
wl_grab -> wl_pointer_grab
wl_input_device_start_grab -> wl_input_device_start_pointer_grab
wl_input_device_end_grab -> wl_input_device_end_pointer_grab