feat: add flatpak build

[vixalien]

Fix #20

Summary of the changes:

• Added a metainfo file (this is the file that's shown to app stores like flathub)
• Added a desktop file (this is the file that allows desktop environments like GNOME/KDE/etc... to show the correct app icon and label)
• Generated the flatpak sources with: flatpak-node-generator npm package-lock.json -o build/generated-sources.json (NOTE: this file will need to be regenerated everytime package-lock.json changes)
• Fixed a few things that flatpak will automatically handle for us (auto-update, registering the protocol handler)

Screenshots:

Next steps:

• Setup CI to build a flatpak archive on each PR and main
• Publish to flathub

[CLAassistant]

All committers have signed the CLA.

[vixalien]

Few issues remaining:

Window activity tracking works, but it doesn't show the proper application names or icons

System Tray is broken (atleast on GNOME)

[Onatcer]

Thanks for the contribution.

are the "Window activity tracking" issues specific to the flatpak build or do you experience it with the packages in the latest release as well? I think in Gnome it should work, in KDE Plasma app icons are not supported (i think app names should work but i might be wrong)