Painter enhancements in WebcamPanel

Hi @sarxos, as per our discussions, those are enhancements I made in WebcamPanel class. I hope all are useful. Additionally I fixed on bug in execution scheduling when panel was in "starting" phase - please see detailed explanation in source code. One other enhancement is in WebcamDiscoveryService - previously you were able to stop it after it run for the first time, but there was no possibility to disable it at all (so it will never start, by intention). Please let me know if you have any doubts in regards of this code.