Simplify winetricks choices

[n8marti]

It seems unlikely that winetricks would be used anywhere other than 1) downloaded by LLI to the INSTALLDIR/data/bin folder or 2) installed at the system level. Attempting to handle an arbitrary path, then, for WINETRICKSBIN could lead to more maintenance effort than is worth it.

Proposal: Only allow two options for WINETRICKSBIN: "default"/"download" or "system". Or, even simpler, leave it undefined for the default value, or accept "system" if the user wants to override that with the system version. In order to be backwards compatible, though, we would need a conversion function to convert legacy paths to either "system" or "default".

[n8marti]

I've discovered that our wine appimage actually includes winetricks. So an even further simplification would be that "default" uses that one, while "custom" could be an arbitrary path given by the user. I think this choice only matters to developers/testers, so I would prefer to not even include it in the GUI, which would just automatically use the appimage winetricks.