statusIconDispatcher: add ibus-ui-gtk to standard implementations

As a stop-gap measure until we have a native input method tray icon,
add ibus-ui-gtk to STANDARD_TRAY_ICON_IMPLEMENTATIONS so that the IBus
status icon shows up in the status area rather than in the message
tray. The message tray location doesn't work for the function of
showing the current input method when switching between windows or
changing input methods.

https://bugzilla.gnome.org/show_bug.cgi?id=641531