desktop-entry: extend OnlyShowIn/NotShowIn to cover session types

Some applications can only work with a specific session type. For example, it doesn't make sense to show a X11 compositor when running a wayland session. Same is true for autostart entries, in fact sometimes autostarting a compositor might break Xwayland.

Can we extend OnlyShowIn/NotShowIn, or perhaps add a new key so desktop entries can be filtered by session type?