window.cpp's WM_CLOSE now calls Close() for wxUniv, else dialogs/
frames won't get closed using system menu/button.
Added missing NULL init for m_artProvider in themes to cure crash
Added 2 sample dsp files for wxUniv (not ideal perhaps but
it will save these getting lost, and much user frustration)
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@15231 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
