Add wxBitmap::NewFromPNGData() for creating bitmaps from PNG data.

This simple function can be used to create a wxBitmap from the raw image data
in PNG format. It is just a thin wrapper around wxImage load functions under
almost all platforms but has two advantages:

1. It can be implemented natively for some platforms (currently only OS X).
2. It can be used in a single expression as it doesn't require creating a
   temporary wxMemoryInputStream and this will be required by wxBITMAP_PNG()
   macro that will be added soon.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72476 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775