Flatpak support

[johan-bjareholt]

Would it be possible to mix these two techniques to build aw-qt into a flatpak package?

http://docs.flatpak.org/en/latest/python.html
https://community.kde.org/Guidelines_and_HOWTOs/Flatpak#Qt_and_KF5_Runtime

[RokeJulianLockhart]

#33 (comment)

@johan-bjareholt, see ActivityWatch/activitywatch#476 (comment).