Flatpak updates - #1636

This essentially matches the stuff currently used on flathub. (Just the source part for the project itself at the end differs)

@rpavlik Thanks! Are you good if we push out 3.2 as it is right now?

i mean, could cope with it, but we'd have to patch in the metadata after the fact again a la : https://github.com/flathub/com.solvespace.SolveSpace/blob/master/metainfo-updates.patch#L66

Best to add the <release version="3.2" date="2025-11-10" type="stable"> type thing to the metainfo.xml.in file before the tag if possible. I'd also ideally like my name fix merged too, but at least this PR updates my email which was in the flatpak files.

For cross reference:
flathub/com.solvespace.SolveSpace#4

flathub/com.solvespace.SolveSpace#5

I'm not going to hit the button just yet, but really by the end of the year 'cause it's way overdue.

@rpavlik oops conflicts.

@rpavlik and please squash them.

@rpavlik ping! :-)

@phkahler I did the rebasing an squashing here #1659.

@rpavlik I hope you are fine and just busy with other things.

Merged #1659