Allow grabbing clients to accept or reject touches any time

Allow grabbing clients to accept or reject touches any time

This is potentially both a performance and client complexity
improvement. An example is a gesture recognizer using touch grabs on
each window with a subscription. If events on a child window are known
to not match any subscription on the child window, then the client
should be able to reject the touch grab even if the parent window hasn't
accepted any of the touches, perhaps because the parent window
gesture hasn't timed out or crossed other thresholds yet.

As an inverse example, the events may match a child window subscription
before the root window has rejected ownership. The child window should
be able to accept the touch proactively. This allows for further clients
to receive a TouchEnd event earlier, and means the client may be able to
reduce state being tracked. If this were not allowed, the client would
need to wait until it received ownership before accepting the sequence.

Signed-off-by: Chase Douglas <chase.douglas@canonical.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>