Icon management cleanup that address the following:
- removed duplicate bitmap field (m_icon and m_dock_bitmap); we keep only m_dock_bitmap
- removed the GetIcon() / SetIcon() interface to keep only GetBitmap() / SetBitmap(); this is more consistent with other wxWidgets API
- addition of the missing legacy wxAuiPaneInfo::icon. It is now aliased to m_dock_bitmap and marked as deprecated.
We can now dig a bit more in icon drawing issues