Support multiple desktop names in XDG_CURRENT_DESKTOP

According to the Desktop Entry Specification, “If $XDG_CURRENT_DESKTOP is
set then it contains a colon-separated list of strings”. For example, on
GNOME Flashback session that variable is set to “GNOME-Flashback:GNOME”.

The value returned by QGenericUnixServices::desktopEnvironment() is in
most cases the uppercase variant of $XDG_CURRENT_DESKTOP variable.

In qgenericunixthemes.cpp, we need to support multiple names in the
return result of that function. If at least one part is in the list of
Gtk+-based desktop environments, then we should use gtk2 platform theme.

Change-Id: I0c9de68756d41b031e822be8cf100ca5c0b202ae
Reviewed-by: David Faure <david.faure@kdab.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>