Refactored Action class to allow for subclassing.

This is in preparation for further work to allow consecutive undoable events to be merged into a single event.
Added API for undo merging.

This allows the actions performed by an UndoContext to be merged with the actions performed by the previous UndoContext. This will be used by the UI to merge drag type actions into a single action for the purposes of undo. By default, no merging is performed - it is only performed if the new mergeGroup parameter to the ndoContext matches the parameter from the previous UndoContext. Merging is currently only implemented for ValuePlug::setValue() calls, but I don't see any use cases for the other undoable actions to be mergeable right now.

This commit also stops the ScriptNode::unsavedChanges() plug getting set when there is no current UndoContext.
Fixed multiple undo entries in virtual sliders cursor up/down nudging.

Fixes #400.
Added ScriptNode::undoAddedSignal().

Fixes #103.
Fixing NumericWidget.valueChangedSignal() return type.

The previous signal type would stop calling slots when one returned True - that's not suitable for a notification that several slots might need.
Fixed multiple undo entries created by color chooser and ramp editor.

- Slider.positionChangedSignal() now passes a reason parameter describing why the positions changed.
- Slider.changesShouldBeMerged() method returns True if two consecutive changes should be made one in terms of undo.
- NumericSlider.valueChangedSignal() now passes a reason parameter describing why the values changed.
- ColorChooser.colorChangedSignal() now passes a reason parameter describing why the color changed.
- ColorPlugValueWidget and RampPlugValueWidget use the above to perform appropriate undo merging.
Updated NumericPlugValueWidget merging code.

The mechanism used by ColorPlugValueWidget and RampPlugValueWidget was much better, so adopting it for NumericPlugValueWidget makes good sense.

- Added NumericWidget.changesShouldBeMerged() method.
- NumericWidget.valueChangedSignal() now passes a reason parameter describing why the value changed (documenting a previously undocumented commit because it needs to be documented in the Changes file for release).
Added a newline for compilers which are partial to them.
Improved undo shortcut for focussed TextWidget based PlugValueWidgets.

Because the TextWidget has its own little undo queue of text edits, it also handles the undo and redo keyboard shortcuts. This was preventing the genuine graph undo/redo shortcuts from being triggered when a NumericPlugValueWidget, PathPlugValueWidget or StringPlugValueWidget had the focus. The shortcuts are now ignored unless there are text editing undo/redo events available, and the PlugValueWidget clear the text editing undos whenever they transfer changes into the plug value. This allows undo to work correctly for both intermediate (not transferred to the plug) text edits in the TextWidget, and for edits to the plug value.
Merge pull request #419 from johnhaddon/undoConcatenation

Undo Improvements
Fixing bug in SplinePlug that caused freezes on CentOs 6