Start to tidy up the code and layout of the mixer window and sliders

Work in progress, so far only supporting vertical sliders.

Use a QGridLayout instead of multiple nested layouts needing
various spacer widgets scattered about to get the layout
consistent over a set of sliders.

Remove a lot of event filtering for context menu and mouse
wheel handling, Qt can now manage most of that itself.  Handle
mouse wheel events by simply redirecting them to the slider.

Remove some questionable installEventFilter(widgetIHaveCreated)
constructs, what was probably meant instead was
widgetIHaveCreated->installEventFilter(this).

Start to introduce QSignalBlocker where appropriate.