Add QProperty to WPushButton for whether the widget is pressed.
Add QProperty to WWidget for the widget value and disabled state.
Add <Text> node for each WPushButton <State>. Allows push-buttons without pixmaps.
Link MIDI input/output ports that have have "in" in the input port and "out" in the output port.
Allow WPushButton to be styled with a style sheet.
Allow WSliderComposed to be styled with a style sheet.
Allow WStatusLight, WDisplay and WKnob to be styled with a style sheet.
Remove duplicate method.
Ensure that audio connections for decks > 2 will succeed on startup.
Merge pull request #134 from ywwg/4deck-audio-input-fix

Ensure that audio connections for decks > 2 will succeed on startup.
Allow WSliderComposed to resize and make the background slider pixmap optional.
Allow setting of QLayout spacing and contentsMargins for WWidgetGroup with Qt CSS.
Use QStylePainter in WWidgetGroup.
Allow setting of QLayout alignment for WWidgetGroup via Qt CSS. Rename other properties to avoid property name clashes by prefixing with "layout".
Add WKnobComposed widget to provide an alternative to pixmap-based rotary knobs.

* Factor out event handling into KnobEventHandler so WKnob and WKnobComposed
  share the same logic.
* Add some more XML utility methods to WWidget.
Fix SVG rendering to scale with higher fidelity.

* Use QSvgRenderer to scale SVG files instead of converting to QPixmap and
  scaling the pixmap.
* Introduces Paintable class that wraps pixmaps and SVG images.
Merge branch 'master' of https://github.com/mixxxdj/mixxx into develop