shell: Don't use the OnlyShowIn/NotShowIn flags from desktop files to hide panels
It was added in 3e04b1c5 to allow downstreams to hide panels that don't make sense in their environment. Later on the ability for panels to hide themselves was added in a78cbe39, which is a better solution for this case.
The current situation is confusing if you are not a known environment, as then no panels show.
Fixes #163 Fixes #45