Dim transit button, instead of removing it when not available

Originally we wanted to to hide the transit mode button in the sidebar when no transit routing provider was available as we didn't have any configured at the time.

Due to a bug in GTK when setting visible on an individual button in a "linked set" not working properly, as a workaround we called destroy() on this button when it should be hidden.

As destroy() will only be supported on top-levels in GTK 4, and the feature has been enabled by default now for a while, we can instead just grey it out in the case when there is not provider available.