AppInfo: use XDG_CURRENT_DESKTOP for OnlyShowIn
Expand the functionality of g_desktop_app_info_set_desktop_env() to
include the possibility of passing strings containing ':' characters (as
some apps, such as gnome-session, are directly passing the value of
XDG_CURRENT_DESKTOP). At the same time, deprecate it, since now we get
the list from the environment variable for ourselves.
Modify the checks in g_desktop_app_info_get_show_in() to deal with
multiple items listed in XDG_CURRENT_DESKTOP. For example, if we find
that we have
XDG_CURRENT_DESKTOP=GNOME-Classic:GNOME
and a desktop file contains:
OnlyShowIn=GNOME
then we will show this file because of the fallback to GNOME. If the
file _also_ contains the line:
NotShowIn=GNOME-Classic
Then we will not show it, because GNOME-Classic comes before GNOME in
XDG_CURRENT_DESKTOP.
https://bugzilla.gnome.org/show_bug.cgi?id=729813