Presets: Add RPCS3 AppImage

[BlueInterlude]

• Flatpak is unofficial/community maintained, RPCS3 team doesn't provide support for the Flatpak

[kencinder]

This doesn't appear to do anything the existing RPCS3 preset doesn't already do, you still have to select the appimage just like you would for a windows executable, and the entire rest of the preset appears to be the same.

[BlueInterlude]

This doesn't appear to do anything the existing RPCS3 preset doesn't already do, you still have to select the appimage just like you would for a windows executable, and the entire rest of the preset appears to be the same.

Would it be better to make it more clear in the windows parser that you can also select the AppImage?

path-to-rpcs3.exe/.AppImage

Something like that?

[kencinder]

We'll leave this open, as discussed in Discord. If you get around to removing exe references in others, add them to this PR. If I get to it first I'll just push to master and close this.