main: Don't override DesktopAppInfo desktop if already GNOME
During the shell initialization we call the (deprecated) function to override the Desktop environment in Gio DesktopAppInfo to make sure that applications are correctly shown (as per commit b2fbf5a2), however this might break the cases in which $XDG_CURRENT_DESKTOP is already set and contains GNOME (given that is now a list).
In Ubuntu this is in fact set to: ubuntu:GNOME. Now, if an application contains NotShowIn=ubuntu, the key will be ignored by the shell, and the application is still listed everywhere.
So, override the DesktopAppInfo desktop environment only in the case that the current desktop is not already GNOME.
At the current date I think we could just safely get rid of this override at all, but there could be still cases where it still might be useful, like when running as nested in some other environment, so keeping it.
(cherry picked from commit a0def239)