Linux | Add Flatpak Support

[tmontes]

STATUS EP 2022 Sprint:

• Hugo has been exploring packaging Mu as a flatpak under Arch Linux.
• He attained a very good level of success.

NEXT:

• He is about to share a PR towards Mu.
• Which will then be adapted to pup.

:-)

[hoh]

Here is my work on Flatpak for Mu: mu-editor/mu#2304