Make wxBitmap::ConvertToDisabled() available in all ports.

Make wxBitmap::ConvertToDisabled() available in all ports.

This method was defined in wxBitmapBase which is not used by wxMSW (and wxOS2)
so it wasn't available there. Move the definition of the method inline and
reuse it for all ports, making it part of either wxBitmapBase or wxBitmap as
appropriate.

This is clearly ugly but we still have no good solution for deriving wxBitmap
from wxBitmapBase in wxMSW as it already inherits from MSW-specific wxGDIImage
there.

Also document that ConvertToDisabled() is only available when wxUSE_IMAGE==1.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66086 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775