Mostly improvements and bug-fixes for the Linux users.
- added automatic starting map selection for MasterLevels
- use -compatmode instead of +compatmode for GZDoom
- make automatic save dir and screenshot dir relative to config dir instead of engine dir (no change on Windows)
- reworked path quoting to fix launching the engine via process output window on Linux
- improvemed the engine's output window
- use the engine's output window by default on Linux
- fixed relative paths of the save directory and demo directory
- record and search demo files in the save directory
- disable gameplay options when joining multiplayer server as a client
- minor bug-fixes and adjustments