GeoStore Login widget changes

Now when yo active the 'adminGUILogin' with the parameters:

* enableAdminGUILogin
* adminGUIUrl
* adminGUIHome
* adminLoginInvalidResponseValidator

into the MapStore configuration file and the user do a normal login (with geostore), other login it's called in background for the Admin GUI and if it's succesfull, show a button to acces to the Admin GUI.

Also, you can change the icon for the admin GUI button with the parameter 'adminIconCls'.