Send idle events from inside wxYield() in all ports, including wxMSW.

This means it can be now done in wxEventLoopBase itself and calls to
ProcessIdle() in the port-specific code are not needed any more, so remove
them.

This introduces a change in behaviour for wxMSW, where idle event handlers
were not invoked from inside wxYield() at all previously, and for wxOSX, where
only a single idle event is now generated from wxYield() instead of a stream
of them until no idle handler needs any more of them as before. But on the
bright side, the new behaviour seems to make most sense and is now the same in
all ports.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@78222 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -17,6 +17,10 @@ Changes in behaviour not resulting in compilation errors
   wxINTERPOLATION_DEFAULT and not wxINTERPOLATION_GOOD as in 3.0 for
   consistency with OS X, call SetInterpolationQuality() explicitly if needed.
 
+- Calling wxYield() in wxMSW now generates wxEVT_IDLE events, just as in the
+  other ports, but this can be unexpected for the applications not expecting
+  their idle handlers to be called from inside wxYield().
+
 Changes in behaviour which may result in build errors
 -----------------------------------------------------