Add wxBITMAP_PNG() macro similar to wxBITMAP() but for PNG images.

Add wxBITMAP_PNG() macro similar to wxBITMAP() but for PNG images.

Just as wxBITMAP() provides a portable way of loading bitmaps from either
Windows BMP resources or embedded XPM data depending on the platform,
wxBITMAP_PNG() hides the difference between loading bitmaps from PNG resources
under Windows and embedded PNG data elsewhere.

Also add wxBITMAP_PNG_FROM_DATA() macro which always loads PNG data from
memory: it's needed anyhow as part of wxBITMAP_PNG() implementation and some
people may prefer to always use it under all platforms.

Finally modify the image sample to demonstrate loading PNG images from both
resources and memory. This involved creation of a new Windows .rc file for it
and copying its data files to Resources bundle directory under OS X.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72477 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775