Fix event handling order in doc/view framework.

Fix event handling order in doc/view framework.

Ensure that the events are always (provided there is an open document)
processed in the following order:

1. wxDocument
2. wxView
3. wxDocManager
4. wxDocChildFrame
5. wxDocParentFrame
6. wxApp

Do this by forwarding the events from wxDocParentFrame to wxDocChildFrame
first and forward them from there to wxDocManager which -- and this part
remains unchanged -- in turn forwards them to the active wxView which finally
forwards them to wxDocument. This requires another condition in the event
handling code as we still must forward from wxDocParentFrame to wxDocManager
itself if there are no active children at all, but this is the only way to
have the same event order in all cases, whether the event is originally
received by wxDocChildFrame or wxDocParentFrame.

Document this and add a unit test verifying that things indeed work like this.

See #14314.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73928 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775