Add ProcessPendingEvents to wxApp::Yield which makes the code almost identical

Add ProcessPendingEvents to wxApp::Yield which makes the code almost identical
to wxMSW's implementation save for using Cocoa instead of MSW.
Add a wxAutoNSAutoreleasePool around wxApp::WakeUpIdle so that calling it from
a thread does not leak the NSEvent.
Add several hundred lines of comments explaining the unique method that
wxCocoa uses to hook the underlying event loop and run loop so that wxCocoa
is able to provide things like wx idle events.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@49237 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775