Add wxWindow::ProcessWindowEventLocally() and use it in wxMSW MDI code.

Add wxWindow::ProcessWindowEventLocally() and use it in wxMSW MDI code.

Add ProcessWindowEventLocally() which wraps ProcessEventLocally() in the same
way as ProcessWindowEvent() wraps ProcessEvent(). I.e. it allows to process
the event in this window only, without propagating it upwards, but taking into
account any event handlers associated with it.

Use the new method in wxMDIParentFrame code in wxMSW to ensure that event
handlers pushed on MDI children frames are taken into account. Add a test for
this to the MDI sample.

Closes #11225.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64262 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775