rmgen doesn't react to alt+f4 / Loading screen for visual replays from command line

[elexis]

Currently one cannot close 0 A.D. with Alt+F4 after starting it from the command line interface before the loading phase was finished.

This would most likely be solved if we had the regular loading screen there instead of not rendering anything in between.

[elexis]

Seems to be the MapGenerator thread which doesn't like to respond to quit requests.

​Phab:P107 kills the thread but segfaults.

[elexis]

In 21264:

Stop the random map generation upon quit request (Alt+F4), refs #4822.

Differential Revision: ​https://code.wildfiregames.com/D1304
Discussed with: Yves

[elexis]

The ParseEntities part after the JS RMS code is still blocking.

[elexis]

In 21659:

Use the loading screen for autostart singleplayer/bot matches and replays too (not only for autostarted multiplayer matches), refs #4822.

Add an autostart-player option, so that one can start as an observer too and watch some knockout bots.

Precise the "Needed for autostart loading option" comment from rP7914 which
become increasingly incomprehensible with each rP17063, rP18355, rP18441 and rP20322.

[elexis]

In 21818:

Fix OSX not starting matches following rP21264.
That commit illegaly polled SDL events from a different thread which for an unknown reason did not error on Windows and GNU/linux.

Refs #4822 / D1304, similarly rP19160.

Differential Revision: ​https://code.wildfiregames.com/D1484
Previous iteration tested on OSX and some comments with regards to letter case by Vladislav (refs rP21817, rP21816).