Add support for the "OnlyShowIn" and "NotShowIn" tags

This is just a proposal because i'm not so happy with the name of the command line option, It's so long. Does someone have a better suggestion ?

Another problem is the break for people using a desktop environment (with XDG_CURRENT_DESKTOP set), because some applications may no longer be displayed.
Might an opt-in option be the better way ?
ditto