Added machinery to Enable/Disable windows. When a window is disabled, it

Added machinery to Enable/Disable windows.  When a window is disabled, it
disables all of its children.  Every window maintains a bool indicating
whether it should be enabled (i.e. Enable() was called on it directly).
When a window is reenabled, it reenables children, but only if they are
actually supposed to be enabled. Override CocoaSetEnabled() in subclasses
to actually enable/disable a Cocoa control.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@23014 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775