Revert "gtktrayicon-x11: Stop using set_double_buffered"

This reverts commit 4a72563c7b801052f4291a4757bd1011946677b9.

It turns out that this commit broke statusicons under Xfce when
not using a compositor.

https://bugzilla.gnome.org/show_bug.cgi?id=737986
Avoid deprecation warnings

We don't really care if we use deprecated API inside deprecated
code, so just ignore the warning.