On some setups (mostly flatpak), the first artist from the
ArtistsView's sidebar may not be selected on launch.
Indeed, it relies on listening to the artists-loaded signal from the
CoreModel to select the first artist once the artists model is
loaded. However, this signal may already have been emitted when the
ArtistView is built. In that case, the first artist will never be
selected.

Now, ArtistsView instead directly listens to the artists model for
changes and uses an initial state variable (untouched_list) to check
if it is needed to activate a row.

The artists-loaded signal from CoreModel is now unused. It can be
removed.