Rename method and update messages.

This method is for selecting multiple elements NOT only curve elements.
The name of the method and the selection messages should convey this.
Rename SelectedElement to SelectedElements
Refactor to remove redundant selection code.
Progress commit.
Progress commit.
Refactor to two base classes.

Selection nodes now inherit from only one of two base classes. The
ElementSelection base class supports multiple selection by holding a
list of uniqueids. In the case of single element selection, this will be
a list with only one element.
Add ISelectionHelper interface.