Add open file option to the -ui graphic

[Massimo-B]

Enhancement or feature request:

Please add an option to "Open" a file from the graphic ui with the default application or additionally "Open with". This would be very useful when comparing pictures or other documents before going to apply the changes.

[gdt]

If you or anyone else would like to write the code, please post the details of the proposed behavior to the hackers list. Otherwise, I'll move this to the not-likely-to-be-implemented features wiki page after a while.

[Massimo-B]

I'm not involved in the development, and don't know about the portability and target platforms of unison... But as for Linux, isn't this just an xdg-open $file ?

[gdt]

Anything we do has to work on all platforms unison runs on. If someone wants to put the time into figuring out a broader plan and propose/discuss on -hacikers, that's fine.

[gdt]

This is similar to but not quite the same as #843.

[tleedjarv]

I could potentially help with the code, given that a specification is provided for how this should work on each platform (I guess XDG, macOS, Windows is what we need). Examples of this working cross-platform in other software could be useful.