Fixing PanZoomBar and Panel issues after #164.

For PanZoomBar, this fixes the slider behavior. Now the buttonclick listener argument also includes a buttonXY property, and PanZoomPanel does not need an Events instance for the zoombarDiv any more.
For Panel, this fixes events for panels outside the map. Just setting the element on the Events instance was no longer enough after e70569b2bb. Events::attachToElement is now used, and it needed to be modified to also work if the Events instance had no element previously.
Finally, I renamed the button property of the buttonclick listener argument to buttonElement, to not confuse it with the browser event button property, and added some more tests and documentation.

tests/Control/PanZoom.html

@@ -159,7 +159,6 @@
         };
 
         var delta, dir;
-        var evt = {which: 1}; // a fake left click
         var buttons = control.buttons;
         map.pan = function(dx, dy){
             t.eq([dx,dy],delta,"Panning " + dir + " sets right delta with slideRatio");
@@ -168,25 +167,25 @@
         //up
         var delta = [0, -50];
         var dir = "up";
-        evt.button = buttons[0];
+        var evt = {buttonElement: buttons[0]};
         control.onButtonClick.call(control, evt);
         
         //left
         var delta = [-125, 0];
         var dir = "left";
-        evt.button = buttons[1];
+        evt.buttonElement = buttons[1];
         control.onButtonClick.call(control, evt);
         
         //right
         var delta = [125, 0];
         var dir = "right";
-        evt.button = buttons[2];
+        evt.buttonElement = buttons[2];
         control.onButtonClick.call(control, evt);
         
         //down
         var delta = [0, 50];
         var dir = "down";
-        evt.button = buttons[3];
+        evt.buttonElement = buttons[3];
         control.onButtonClick.call(control, evt);
         
         map.destroy();