Properly support WINE

[probonopd]

Properly support WINE and Windows executables (in case it isn't by default). We need to think about what this might entail.

[probonopd]

Treat .exe like applications, similar to how we treat application bundles and (as a fallback) .desktop files.

When launching them, construct an environment with the WINEPREFIX they are located in, so that launching an .exe in fact results in it being launched using the correct WINE and the correct WINEPREFIX, even if there are multple ones on the system.