[Feature request] Add Flatpak and snap support

[kacperpaczos]

I would like MintSources to have two new tabs: Flatpak support and Snap support.

The Flatpak tab should have options for managing repositories: viewing current repositories, adding new ones, and enabling beta and nightly options from Flathub. It may also include a solution for installing beta or nightly versions of GNOME programs if they are available as flatpak.
https://wiki.gnome.org/Apps/Nightly

The Snap tab should have a graphical interface for managing snaps and, until MintInstall supports this format, allow one-click installation of the Snap Store.
https://snapcraft.io/snap-store
sudo snap install snap-store --channel=preview/candidate
https://github.com/ubuntu-flutter-community/software

[haggen88]

@kacperpaczos Mint's policy on Snap: https://linuxmint-user-guide.readthedocs.io/en/latest/snap.html

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