ibusCandidatePopup: Make pagination buttons clickable without stealing focus

The default StButton event handlers will create a temporary grab that will steal the keyboard focus out of the client, and cause it to pop down the candidates window.

Avoid the default grabbing behavior, so that focus stays on the Wayland client while interacting with the ibusCandidatesPopup with the pointer.

Closes: mutter#2244 (closed)