Added new cleared event that can be bound for when user clears the document, updated Wave gadget code to handle it

git-svn-id: http://svg-edit.googlecode.com/svn/trunk@117 eee81c28-f429-11dd-99c0-75d572ba1ddd

editor/svgcanvas.js

@@ -580,6 +580,7 @@ function SvgCanvas(c)
 				i++;
 			}
 		}
+		call("cleared");
 	}
 
 	this.getMode = function() {

wave/svg-edit.xml

@@ -232,6 +232,7 @@ SVG-edit Google Wave Gadget @ <a href="http://svg-edit.googlecode.com/" target="
 			wave.setStateCallback(stateUpdated);
 		}
 		svgCanvas.bind("changed", sendDelta);
+		svgCanvas.bind("cleared", stateUpdated);
 	}
 
 	gadgets.util.registerOnLoadHandler(main);