Delimited text to core

This should implement getting the delimited text moved from plugin to core. 

Only one thing I'm not sure about in terms of implementation approach, which is adding another signal to Qgis app, addSelectedVectorLayer, which is directly equivalent to addVectorLayer.  Not sure whether there is any reason not to make addVectorLayer(QString,QString,QString)  a slot and bypass this additional function, but chose not to change this...