Use consistent prefix and portal name

The xdg-desktop-portal-phosh should be name phosh in the portal config but that was already taken by phosh itself which provides the access portal. So in order to name the portals provided by the shell itself phosh-shell.portal we want to switch the DBus names to mobi.phosh.Shell too to be consistent. That prompts for moving more bits into this domain (to be consistent again) which ended up in this series moving also the desktop, session files, etc. Note that we didn't move the GSettings as that would break existing configs.

Since we want the correct version relationships we bump the version to 0.44-dev already.

Needs to be merged in sync with guidog/xdg-desktop-portal-phosh!9 (merged).

/cc @arun-mani-j could you have a look, would be nice to merge that quickly to give it solid testing before the release and to not trip up too many MRs.