Don't generate duplicated events for clicks on tree +/- buttons.

In multiple selection mode we process mouse events manually in wxTreeCtrl
itself and already take care of mouse clicks on the buttons there so don't
pass them on to the base class which would generate another event.

Closes #11098 (thanks to Jonathan Liu).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61783 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775