keybindings: Add two extra checks for key grab binding
display: Make sure to ungrab the pointer before starting a grab op

Now that we grab devices on the X11 connection, we can run into
cross-connection issues. Since GTK+ frames are on the UI connection,
they'll get the passive grab when we click on them. Forcibly ungrab
on GTK+'s connection before attempting to take a grab on the backend
connection ourselves.
display: Simplify cursor theme handling
display: Set the cursor theme on the backend display as well
keybindings: Remove two extra error traps
keybindings: Simplify
compositor: Also ungrab the VCP when beginning a modal

XXX: This should not be needed. We need to figure out why
there's a grab on the UI connection.