auto-update support in appimage

[soredake]

Describe the bug
I know that syncplay is rarely gets new releases now, but it's still will be nice to have auto-update support in appimage.

To Reproduce
Steps to reproduce the behavior:

1. Download some old version of syncplay appimage.
2. Launch it.
3. AppImage can't update itself to newer version.

Expected behavior
Syncplay will auto-update when using a appimage version and new version is out.

Screenshots
If applicable, add screenshots to help explain your problem.

Version and platform:

• OS: Linux
• Syncplay version and build type: Syncplay 1.6.8
• Media player and version: MPV

Additional context
https://docs.appimage.org/packaging-guide/optional/updates.html#making-appimages-self-updateable
https://appimage-builder.readthedocs.io/en/latest/advanced/updates.html

[soredake]

I agree, as alternative, syncplay can just download newer appimage, make it executable, close old version and launch newer (that's what rpcs3 is doing in linux appimage IIRC).