Fix tree control focus issues on OS X (and possibly others) caused by toplevel.cpp...

Fix tree control focus issues on OS X (and possibly others) caused by toplevel.cpp not giving focus if children eat the LEFT_DOWN event... added event.Skip() on LeftDown() (whether an item is hit or not) after all dragging and moving conditions have been considered so that LEFT_DOWN events are never eaten by the tree preventing receiving focus

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@30581 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775