xdg-autostart-generator: Add a generator for XDG autostart files

xdg-autostart-generator: Add a generator for XDG autostart files

This generator can be used by desktop environments to launch autostart
applications and services. The feature is an opt-in, triggered by
xdg-desktop-autostart.target being activated.

Also included is the new binary xdg-autostart-condition. This binary is
used as an ExecCondition to test the OnlyShowIn and NotShowIn XDG
desktop file keys. These need to be evaluated against the
XDG_CURRENT_DESKTOP environment variable which may not be known at
generation time.

Co-authored-by: Henri Chain <henri.chain@enioka.com>