Mapview: option to hide unclinched segments

[jteresco]

From user julmac by email:

"One additional feature I would find useful is the ability to completely turn off un-driven segments; that way I could see a map of my travels (on a basic base map) without any additional clutter."

One way to implement might be as part of #350, using completely transparent segments for unclinched.

[jteresco]

This came up again in a forum post by a new user: https://forum.travelmapping.net/index.php?topic=5668.0

Additional thought: the way to do this is not the completely transparent segment idea mentioned above. It would need to be an option to load only those clinched segments from the DB. The UI is not hard, it's just figuring out the right SQL query to get the info we need and do so efficiently.