support appimages

[DetachHead]

Is your feature request related to a problem? Please describe.
appimage files support update metadata however to actually be able to use it, you need to rely on a third party tool such as gear lever. it would be nice if this was supported natively in linux mint's update manager.

Describe the solution you'd like

• in mintupdate:

  • include available updates for installed appimages based on the update information embedded in the appimage.

• in mintinstall:

  • appimages should be installable when double clicked (similar to .deb files). this will place them in the app menu just like gear lever does
  • once installed, they could show up in the "installed applications" list so that users have an easy way to uninstall any of their applications from one spot

(note that i am not suggesting that appimages be searchable via the software manager, see additional context below)

Describe alternatives you've considered
just use gearlever. but it's convenient to have all updates managed by one tool

Additional context
appimage support was previously suggested in linuxmint/mintinstall#214, but it was pointed out that there's no central appimage reporitory that contains direct links to the appimage files. this is not what i'm requesting. as pointed out in linuxmint/mintinstall#214 (comment), simply having appimages supported by the update manager would be sufficient