Skip mouse button release events in wxGenericTreeCtrl.

Not doing this prevented the default handling from taking place resulting in
internal confusion in GtkNotebook when wxTreeCtrl was placed inside it: the
code there set the button being held by user in its mouse press event handler
and reset it in its mouse release event handler which was never called because
we didn't skip the event, resulting in ignoring the next mouse press in the
notebook.

Closes #16055.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@76093 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775