Applied patch [ 679397 ] remove GPL code from motif build

  Applied patch [ 679397 ] remove GPL code from motif build
from Ian Brown, with some modifications; this patch replaces the (GPL)
Motif ComboBox with an implementation based on the native (for Motif 2.0)
ComboBox. The old ComboBox is still used as a fallback for older Motif
versions. My modifications include some method's implementation and
some workarounds for LessTif.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@19114 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775