Flatpak CI

This is a first attempt at implementing Initiatives#1 (closed) for Meld. The CI-generated flatpak appears to do the right thing in my limited testing, though it could certainly use some improvement. It should theoretically be parallel-installable with a non-nightly flatpak, though this is untested.

The one obvious thing we're missing in the nightly-build setup is a nightly-specific icon.