Utilized a better approach to detect if a class is already present
Resolves #13

We can now deselect a class!!!

The ApexNavigator now has a new method "removeUMLClass(apexClassName)".
This method will remove the selected class from the various holding
arrays and then update the canvas display.  This method was modeled from
the Canvas2D.Sheet.clear() method (see method in the
Canvas2D.standalone.js file; appox line 6335).  Also, that
Canvas2D.Sheet.clear() method can be used to setup a "Clear All" button
on the ApexUML app!!!