Make the dial widget more compact

The dial widget no longer requires the space which was previously left
empty at the bottom of the dial.
Add help bar to motor panel
Add a controller parameter to panel constructors

The Controller class that is using the panel is passed through this
parameter.
Add red stop button to Controller window

This button appears when one or more motors are running (whether or not
they have been set running by the Controller). When the left knob is
pressed, the button on-screen appears depressed too. This is to provide
a hint that the left knob stops motors when pressed.