Make progress track creation folder configurable

[cwegrzyn]

Fixes #46

Adopts the following pattern which we can eventually use for other creators if we like it

• You can set a "default progress track folder" in the settings. This defaults to the old value of "Progress"
• In the Progress track creation modal, you can pick an alternative folder to put this particular track into. I thought this might be helpful if you wanted to organize your tracks by, i dunno, type or location
• In any event, if the folder you specify does not exist, it will be created