qmlui: fix RGB matrix editor color selection
qmlui: fix RGB matrix editor color selection

Reported: https://www.qlcplus.org/forum/viewtopic.php?f=35&t=15871

mcallegari/qlcplus,mcallegari/qlcplus,mcallegari/qlcplus,mcallegari/qlcplus,mcallegari/qlcplus,mcallegari/qlcplus,mcallegari/qlcplus