Add support for template NSImages to wxBitmap

wxBitmap uses CGImage instead of NSImage internally and the conversion
looses NSImage metadata.  In particular, it looses the "template"
attribute, which is set for files ending with "Template" and loaded
trough wxArtProvider.

This change makes it easy to use template images with native controls
such as the toolbar.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@78017 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775