Add Flatpak support

[bordam]

This is my attempt to package the app in flatpak, an universal linux package format.
I tried to follow these guides as closely as possible:

• https://docs.flatpak.org/en/latest/electron.html
• https://docs.flathub.org/docs/for-app-authors/requirements
  • I couldn't find info on how to build the app, so I did what popular projects like freetube did, I repacked the deb without building from source. But this means no aarch64 build. It could be made so that the release also publishes an aarch64 deb and it would be easy to package for that architecture as well. See https://github.com/flathub/io.freetubeapp.FreeTube/blob/master/io.freetubeapp.FreeTube.yml . I think this could be eligible for an exception with flathub requirements, as I expect it would be much more complex to build the app from source and other popular apps do that.
• https://docs.flathub.org/docs/for-app-authors/metainfo-guidelines
  • Info could be added about the compatibility of the app with smaller devices etc. Also we could add info about the minimum size of the screen
• https://docs.flathub.org/docs/for-app-authors/metainfo-guidelines/quality-guidelines
  • I couldn't take better images than the ones I've taken, (the settings are long and it's good to have them all shown in one screeenshot; the main window can't be resized less than a fixed width) altough they are a few pixel larger than what's requested by the quality guidelines

Now that I also added an appstream metainfo file, it would be great to introduce it also in the deb and the rpm! I'll try to do that but I'm not sure how to do it

From my testing the flatpak works great. Only issue I've encountered is the app indicator, like Ferdium flatpak it technically works but the right-click menu is displaced from the appindicator, and the theme is not GNOME's theme. But I think this is a problem that doesn't depend from us, must be some flatpak or libappindicator issue.

For testing purposes, the metainfo currently takes images from my repo https://github.com/bordam/webeep-sync-flatpak, we'll change that to the new images I inserted in the repo when this PR is merged.

If this works well and we end up merging, it could be published on https://flathub.org and be easily discoverable and installable on software stores! Most famous linux distributions support flathub out of the box! Linking this for future reference: https://docs.flathub.org/docs/for-app-authors/submission

[bordam]

To sum up, these are the next steps:

• Either build an aarch64 deb in the release workflow or change the flatpak to build the app from source
• Update the metainfo with device/screen size compatibility info
• Improve screenshots [optional]
• Integrate the metainfo in deb and rpm packages
• Merge
• Change screenshots' links in the metainfo
• Publish on Flathub: https://docs.flathub.org/docs/for-app-authors/submission
• Verify on Flathub: https://docs.flathub.org/docs/for-app-authors/verification