Better fix for duplicate wxContextMenuEvent generation under MSW.

Better fix for duplicate wxContextMenuEvent generation under MSW.

Fix the bug with multiple wxContextMenuEvent being generated for a single
WM_CONTEXTMENU without breaking context menus for wxTextCtrl (and all the
other native controls). Do this by ensuring that WM_CONTEXTMENU is still
passed to DefWindowProc() if we don't process it instead of just being eaten
completely in any case.

Also add a unit test checking for this bug to ensure it stays fixed.

See #13683.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74329 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775