Def ctor for knob + logscale patch

This contains two things. I am sorry, but I accidently merged these two. The patch contains:

   * Default ctor version for knob class
   * Extensions for knob (requested by @diizy)
   * QColors depointerized (requested by @tobydox)
   * Logscale Patch, including fix suggestions by @tobydox

The patch contains two minor bugs: Stereo linking does not work for logscale connections (@tobydox promised to fix this) and the pointer in the automation window always shows linear values to the user.