Allow SWT cursor tools to be triggered only by specific mouse buttons

Requests for enhancement

• org.geotools.swt.event.MapMouseEvent: make the underlaying
  MouseEvent accessible by a getter

• sub classes of org.geotools.swt.tool.CursorTool: process
  only onMouse*-function if the first mouse button is used.

Rational:

I have embedded SwtMapPane into a RCP view and added a context
menu to SwtMapPane. If a cursor tool is active and one clicks
with the right resp. the context menu button to show the popup
menu the action of the cursor tool is triggered too.