WIP: Feature: Linux users could not run Mu using Flatpak

[hoh]

Solution: Add a manifest file for flatpak-builder

This is work in progress from the EuroPython 2022 Sprints in Dublin.

I took some inspiration from the following Flathub GitHub repositories:

• https://github.com/flathub/cc.arduino.arduinoide
• https://github.com/flathub/com.jetbrains.PyCharm-Community

Known issues:

• Mu does not detect device vendor and properties, failing to connect via serial (overriding the code makes it work).

Submission on FlatHub

Once the team is happy with the state of this pull request, the manifest should be submitted to FlatHub, the most popular Flatpak repository, for all users to discover using their favorite Linux distribution.

Integration with Pup

I discussed with @tmontes about integrating this with Pup. It would be nice to have Pup generate the Flatpak manifest and organize related files automatically from a multi-platform project description.

[hoh]

In order to be published on Flathub:

Accessing the network is not allowed during the build process, to download extra files, everything that an app requires must be included within the manifest. There is a community provided set of tools to make this easier for some projects (such as npm) that would traditionally want to access the network.