ENH: add Markups text display setting

Working on the ruler display node settings it became
obvious that the text display toggle belonged on the
Markups display node superclass.
Moved GetMarkupsDisplayNode to the markups node superclass.
Added TextVisibility to the markups display node, testing.
Added defaultTextVisibility to the settings code.
Updated the fiducial displayable managers to use the text
visibility flag.
Added the ruler nodes to the drop down in the GUI, set ruler positions
from the table, generalised some code.
Fixed a typo in some ui element names, added the text visibility
push button.

Issue #3367