Flatpak: Make it build with GNOME Builder

The in-tree flatpak manifest is only useful in case it is used by the CI or by GNOME builder or some other IDE with Flatpak support. The changes in this merge request makes it possible to build and run Evolution from GNOME Builder directly.

There were some probably undesired changes in this merge request like removing the news to appdata calls, i don't think it useful to have as it is a releasing part and not something that makes sense to have for development stages and also can't be made to work with GNOME Builder as it has no idea which sources to run as scripts and when because it can't know which should replace with the "current tree"