Added ColorSwatchPlugValueWidget.

This displays a colour plug purely as a colour swatch, without the numeric fields. It supports the dragging of the colour from the swatch to other plugs and the script editor. The ColorPlugValueWidget now uses it for its swatch, so that too supports dragging now.

Fixes #625.
Added Style::changedSignal().

This is emitted when some aspect of the style has been modified, and is used by the Gadget to emit Gadget::renderRequestSignal(), triggering a redraw.