Draft: Private flatpak install

Making a MR so people can start having a look :) Also, I've got a few local changes I want to send in, and a commit to test so that helps me keep a clean work tree :)

TBD:

• write proper commit message
• the appstream data fetching happens synchronously when opening a project. Make it async and hook the flatpak transaction to the UI to get progress report.
• We need some way of keeping the appstream data and the runtimes updated automatically, and have a way to trigger it manually too. I like opening up the computer and seeing that an annoying bug disappeared with automatic update :)
• We'll need to tweak the flatpak export phase so that it provides instructions to install the appropriate remotes (gnome-nightly, flathub-beta, flathub) on the target system. For example a dialog window with those instructions and explanations. Might be worth checking if the main gnome-nightly wiki should include that too.
• remove all debug messages (g_warning()s)
• probably other stuff I forgot about :)

Fixes #1486 (closed)