Adapt to IBus socket path changes

Since https://github.com/ibus/ibus/commit/8ce25208c3f4adfd290a032c6aa739d2b7580eb1, IBus will prefer the WAYLAND_DISPLAY environment variable content to create a socket filename, in order to have IBus daemons and clients talk through this private bus. 

Qt reimplements the code that finds the IBus socket file, thus needs updating to the same logic in order to talk with IBus properly. Please find attached a patch that should make the IBus-based input method use this newer path if the socket file exists, and fallback to the traditional path otherwise.

This should cater for Qt apps (doesn't matter if x11 or wayland backend) in both x11/wayland sessions.