Improve the "Test map" feature

[ioan-chera]

Make sure "Test map" can load the following:

• [x] macOS bundles
• [x] open macOS bundles just with system, no forking
• macOS: give it a Spotlight search for any apps (out of scope)
• maybe even vanilla DOOM via DOSBox (out of scope)
• [x] Also make sure user can go back to it in case they want to change the port path.
• [x] Linux: make it start the game independently
• Linux: improve the GUI by letting user write the app name. Try to auto-find it (out of scope, address later).