MainWindow: use settings.create_action

[danirabbit]

• Use Settings.create_action to manage syncing the gsetting and action state. There's no transforms needed here
• Make settings bind to the map GET only. We don't need to update the gsetting if the map source changes because the only way to update it is via the setting. This can be a one-way relationship where the map always follows the gsetting
• Rename the string constants to be clearer that these constants reflect the gsettings names, not the IDs of map sources
• Remove a bunch of binds we don't need

I still kept the transform instead of making the enum reflect map source IDs because we're going to want to keep those separate for dark mode. That way the setting is always explore or transit and then in the transform func we can figure out if that means explore-dark or explore-light etc.

If dark mode is set, we can change the map source from explore-light to explore-dark and that won't trigger a write to gsettings since the user setting didn't change here, just the name of the map source we're currently using

[ryonakano]

Leaving two tiny comments (and one self-talk)