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
Since we have to use pkill, kludgily, for the right combination of
portability and featurefulness, put the code in one place rather than
duplicating it everywhere.
https://bugzilla.gnome.org/show_bug.cgi?id=635089
gnome-volume-control-applet was renamed to gnome-sound-applet when
moved to the control-center module, so we need to check for both names
when identifying the legacy status icon.