Allow marking a panel that it should hide on resize

This update allows a panel to have an additional CSS class that it should be hidden on resize. Resizing via a split bar now adds the wcResizing class to the frame. In this way, you can combine the two to hide the content when resizing a panel. This does NOT currently take browser resizing into account, only split bars. For example:

    <body>
	<button>Add Panel</button>
	<div id="container"></div>
	<script>
	var docker = new wcDocker($('#container'));
	docker.registerPanelType('hidden',function(panel){
		panel.layout().addItem($('<p>I should be invisible on resize</p>'))
		panel.resizeVisible(false);
	})
	docker.registerPanelType('visible',function(panel){
		panel.layout().addItem($('<p>I am always visible</p>'))
	})
	docker.addPanel('hidden',wcDocker.DOCK_LEFT,false)
	docker.addPanel('visible',wcDocker.DOCK_RIGHT,false)
	docker.addPanel('visible',wcDocker.DOCK_LEFT,false)
	docker.addPanel('hidden',wcDocker.DOCK_TOP,false)
	$('button').on('click',function(){
		docker.addPanel('visible',wcDocker.RIGHT,false)
	})
	</script>
    </body>