add DisplayName setter

When You add new WmtsLayer by default it has empty label on legend
control. To avoid this issue add setter for DisplayName when
CurrentLayer changing.
set all WMS layers visible at startup
handle invalid GetCapabilities XML format

Set InitializationFailure rather than throwing an unhandled exception
when OGC service response is not valid GetCapabilites XML format:
http://v2.suite.opengeo.org/geoserver/wms?request=GetException&