full virtual desktops show too many applications in their menus

[Antoine Martin]

We filter out the ones that are not to be used in virtualized environments in load_desktop_menus.py, but when we launch a full virtual desktop it will use all the apps found in /usr\[/local\]/applications

We need a way to tell those window managers what .desktop files to use... and more importantly, which ones to ignore.

Create a new $XDG_CONFIG_DIRS=~.shifter/server/virtual-desktop-menus for the virtual desktops and set the environment variable before launching them.
see ​xdg menu spec and ​environment variables

We must create this local copy on first startup (which means it will go stale... sigh) and patch it to remove the entries we don't want.. (tricky - see #94)

See also bug #43 (remove shutdown option from menus)