issue #44: remove textCapCharacters from key input widget

Keys are mixed case so this just makes it difficult to enter keys.
add Session.clearStations()
change visibility of some of the preference key constants
use a listener to respond to preference changes

The listener will force the default session to update the active credentials
when they are changed and clear station data when it is stale (issue #43).
Merge branch 'issue/43' into develop