Added sensor address range qualification

Limits addresses between 1 and 4096
UI to clear trains from Dispatch Panel

Clear buttons added to train list section of dispatch panels that will
clear out the train id for the associated train label.  Buttons are
hidden if no train is associated with the label. (Also eliminates
confusing Safari workaround in prompt dialog.)
Merge pull request #43 from svlPanelMaker/master

Sensor address range qualification and GUI to clear trains on dispatch panels