Eliminate unnecessary widget renders from control changes.

Instead of re-rendering on every control change (via
WWidget::onConnectedControlChanged), check for changes in what we would render
first.

Updated: WDisplay, WKnob, WKnobComposed, WSliderComposed, WStatusLight
Left unchanged: WWaveformViewer, WPushButton, WOverview, WVuMeter
Merge pull request #222 from rryan/wwidget-update

Eliminate unnecessary widget renders from control changes.