Reduce diff noise (no functionality changes)
ITabPage2 can be implemented in application-specific code
Widget.Initialize() should never be called directly.  It is only called via the constructor.
Reduce more diff noise
Update MonoMac to latest (with a few tweaks)
Update InstanceWidget.Initialize to apply default styles if any
TabPage default constructor is needed for JSON for some reason.  Having a default value does not work.
Eliminate warning
Fully implement ImageMenuItem.Image, ToolBarActionItem.Image, and BaseAction.Image
Mac: Added new options to ToolBarItemHandler to use a button-style toolbar item, and convert image to grayscale if desired (using styles)