Moved widgets into subpackage
Added settings util class

Added Settings class to make reading and writing settings easier for
plugins. THere is quite a bit of overhead using the getitem/setitem
functions, but settings should not be used so often, hopefully.
Added settings attr to Plugin

Also started using it in PCA with ExRememberPrompt.
Cleanup and fixes
Added get_or_prompt Settings util

Added utility function to get a setting, or prompt for it from a list of
options. Also adds a checkbox to the prompt so the user can ask for the
setting to be stored ("Remember this setting").