Proper handling of ELF files in desktop environments

__If you care about this issue, then please open an issue ticket with your favorite desktop environment including a backlink to here, and add the URL to the ticket you have opened to this thread.__

- [ ] GNOME https://gitlab.gnome.org/GNOME/nautilus/issues/443
- [ ] KDE https://bugs.kde.org/show_bug.cgi?id=394672
- [ ] MATE
- [ ] XFCE
- [ ] Elementary
- [ ] Cinnamon
- [ ] LXDE
- [ ] ...

Currently our users have to set the executable bit before they can run an AppImage. This apparently is a real barrier for them.

Desktop environments should ask the user whether to set the executable bit and then run the application when the user double-clicks on an [ELF file](https://en.wikipedia.org/wiki/Executable_and_Linkable_Format) that doesn't have the executable bit set.

This is in no way limited to AppImages but also applies to other types of executables.

Vaguely similar to this dialog that Firefox brings up:


This is what Linux MINT users currently see, which is certainly not ideal:


It should instead ask "do you want to mark this application as executable and launch it?".

EDIT 18.05.2018: In https://gitlab.gnome.org/GNOME/nautilus/issues/443 this has been proposed for GNOME Nautilus: