fix cursor in IE after dragging a ext-window

solve https://github.com/camptocamp/cgxp/issues/802
bug in Extjs 3.4.x
http://www.sencha.com/forum/showthread.php?264999-Ext-JS-3.4.x-IE10-Move-cursor-remains-after-dragging-Window

Perhaps only do it for IE, not for other browsers?