Handle successive key presses better in wxGenericTreeCtrl search code.

Handle successive key presses better in wxGenericTreeCtrl search code.

Go to the next item starting with the given character if the same one is
pressed multiple times. This is more useful than searching for an item
starting with multiple occurrences of this character (which usually won't
exist) and is more consistent with how Windows handles this.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72636 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775