menu: Add Endless OS desktop to OnlyShowIn environments list

It’s used to identify Endless OS desktops, which use a modified version of GNOME.

It’s always used alongside GNOME (i.e. DesktopNames=endless:GNOME), but is useful as a separate identifier to allow distinguishing between modified Endless OS desktops and stock GNOME.

It’s defined in /usr/share/wayland-sessions/endless-wayland.desktop on Endless OS systems. That file is provided by the gnome-session package whose Debian packaging data is modified during packaging. (i.e. There’s no git repository I can point you to which defines the endless desktop ID.)

Signed-off-by: Philip Withnall philip@tecnocode.co.uk