sidebar: Grey-out transit switch when not available

Marcus Lundblad requested to merge wip/mlundblad/always-show-transit-button into master

Due to a bug in GTK where setting visible false on a linked button not working properly, we used to call destroy() on this button when transit providers where not available. Since calling destroy() on widgets will not be supported in GTK 4 and we could instead dim the button in this case as this feature has now been exposed by default for a while. And originally it was hidden since the feature where itself hidden.

Fixes #361 (closed)

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