Generate wxEVT_KEY_DOWN events for navigation keys in wxMSW wxTreeCtrl.

Generate wxEVT_KEY_DOWN events for navigation keys in wxMSW wxTreeCtrl.

We only generated wxEVT_COMMAND_TREE_KEY_DOWN events for the keys which can be
used for tree control navigation and which are handled specially in the code
but not the wxEVT_KEY_DOWN ones. This was inconsistent with the other keys and
incompatible with wx 2.8 and other ports, so generate both events unless the
wxEVT_KEY_DOWN one was handled by the user code.

Closes #11621.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63169 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775