wayland-server: Add touch grab support

Touch grabs allow the compositor to be placed into a mode where touch events
temporarily bypass their default behavior and perform other operations.
Wayland already supports keyboard and pointer grabs, but was lacking
corresponding touch support.  The default touch grab handlers here contain the
client event delivery code that was previously called directly in weston.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>