Performance improvements, 2 bug-fixes, more defaults

• Fixed a bug related to the globalAlphaMultiplier attribute.
• Fixed a bug related to centerOn methods and some Viewports.
• Performance improvement: Events (Callbacks/Highlight/Selection/etc.) are now Pooled.
• Added a default Color (black) for the following PieWidgetStyle attributes: separatorColor and circumferenceColor.
• Added a Testing Menu so that Contributors can more easily test their implementations.
• Added the NO_SELECTION constant in PieMenu.
• Added a new "code example": NestedClickDrag.
• Minor JavaDoc tweaks and improvements.
• Updated ShapeDrawer version to v2.3.0.