Cleaned up mouse event tests
Condensed mouse event code (it's not an improvement)
Restored previous mouse event functions and simplified them slightly
Working on preventBrowserDragDefaults()
Got HtmlElement.preventBrowserDragDefaults() working
Modified client.js to use HtmlElement.preventBrowserDragDefaults()
Added HtmlElement.isBrowserDragDefaultsPrevented()
Simplified tests of default prevention in _client_test.js