Added FloatProperty and ChoiceProperty.

Added FloatProperty and ChoiceProperty.

ChoiceProperty will be used for 'curve', 'playlist', and possibly 'driver'.

We'll group the outstanding types following:
  PathProperty():
      5 file
      2 path
  To be renamed:
      1 glob   -->   string
  To be determined:
      3 object
      3 dict
      1 function
  Handled by callbacks, to allow clicking on plots:
      1 point