We were always showing the starred item in the sidebar, with an empty
state in the resulting view if no starred file was present.

This is usually what we want for regular items. However, in 3.28 the
feature is not working as good as we expected for those not using
the tracker directories as heavily as other users.

In order to prevent being too annoying, we can hide the sidebar item if
no starred files are present, while still providing the context menu
to be able to use the feature.

This can be reverted once we are able to star any file, once tracker has
the required API to star a file and have a sync callback for its result.

Closes: #338