update appdatafile

[Murmele]

• Update version in the appdata file Currently changelogs are written down in two places, which makes it quite error prone. At gittyup we are creating from the changelog.md file the xml release information and then we create out of it the appdata file:

for that we are using cmark. cmark creates a html version of the changelog https://github.com/Murmele/Gittyup/blob/master/src/app/CMakeLists.txt#L42-L50

The html version is then used and placed into the appdata file: https://github.com/Murmele/Gittyup/blob/master/cmake/generate_appdata.cmake

Just a recomandation for writing the changelog in a single place and distributing it multiple times.

[Murmele]

@chipmuenk Can you enable the flatpak build. It shows me up that it was disabled manually