ENH: add the ability to add custom actions and trigger them

Added a list of actions that are added after the default none,
add, remove, rename actions that the combo box supplies. When
the new action is selected, it is triggered so a module can
add a connection to listen for the triggered signal on the action
and activate a local slot.
Added a test to exercise trying to add duplicate or conflicting
actions.
Use the foreach construct to iterate over the user actions.
From development on new Markups module.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22136 3bd1e089-480b-0410-8dfb-8563597acbee