ENH: add a reset to default display properties button

Add a button to the modify display properties collapsible group box
to allow the user to reset the display properties of a markups node
to the default values.
ENH: add persistent saving of default display node properties

Use the ctk settings on the application to save default
markups display node properties across executions of Slicer.
TODO: debug why the settings panel interacts poorly with the saved
settings in the logic class.
Added a button to save defaults in the GUI.
Added testing.
Added an event so that a markup added via mouse click will have
it's display node set up from the defaults.
STYLE: fix formatting for eventual rebase

Tab to spaces, indent updated from mrml node combo
box branch changes to avoid conflicts.