compositor: Don't assume we're a Wayland compositor
cursor-tracker: Don't assume we're a Wayland compositor
display: Set the cursor when calling XIGrabDevice again
display: Correct the detection of a grab

The grab_window might be NULL, in which case we have a full-screen
grab, but we might still in a grab. Correct the check by asking
whether we're in a grab op or not.
Support X button events again

Do this by duplicating the current code and porting it to use
X again. A better approach would involve our own event structures,
and I really don't want to do that right now. We can clean this up
later.
Support keybindings again, too