Linux: configure wine prefix for ScriptMerger

[basxto]

Allow to define a wine prefix (path) that’s used when calling ScriptManager through wine.
A special prefix could be used to trick ScriptManager into launching native kdiff3.

[t0mtee]

If you run W3 through Steam, you can do this manually by editing /src/gui/main_widget.py if you have protontricks installed - replace line 1082 with:
subprocess.Popen(["protontricks-launch", "--appid", "292030", scriptmergerpath], cwd=directory)
If you run W3 as a non-Steam game, change the appid to whatever your custom appid is (can be found through protontricks GUI).