Add OpenArena autodetection

[Calinou]

I'd like to be able to extract Quake3 binaries into an OpenArena installation (with its baseoa folder) and have it launch automatically, without having to specify launch parameters through a start menu option or similar. Could this be implemented by checking for the presence of a baseoa folder if no baseq3 folder is found in the CWD?

For reference, these are the launch options I use to make Quake3e fully functional with OpenArena and its mods:

+set fs_basegame baseoa +set protocol 71 +set sv_master1 "dpmaster.deathmask.net" +set sv_master2 "" +set sv_master3 ""

[xplshn]

Check this out: https://github.com/flathub/io.github.ec_.Quake3e.OpenArena/blob/master/openarena-q3e

Its the launch script used for the Flatpak I maintain (openarena quake3e - https://flathub.org/apps/io.github.ec_.Quake3e.OpenArena)