Prevent KisWdgGenerator from calling setConfiguration multiple times

This prevents KisWdgGenerator from calling the setConfiguration method
of the generator multiple times. KisWdgGenerator was calling setConfiguration
with the default configuration followed by another call with the stored
configuration.
In the case of the screentone generator it caused that when opening
the layer options dialog it rendered with the default configuration inmediatelly
followed by a render of the correct options, showing a fast "flash" of the
image provoked by the fast double render of the different configurations.