Remove VCSlider::m_tapButton & m_timer

These are not used, they were used for submaster mode of the slider, that was removed long time ago.
It's better to remove them now, and add them back again when submasters are added again, than have this
unused code here.
Speed dial feedback fixes:

- update feedback when starting Operate mode
- update feedback when the speed is changed via tap button
Document "next" feature of the tap button.
Merge pull request #176 from janosvitok/master

Remove VCSlider::m_tapButton & m_timer