Add backward compatible file export

One strength of blender has been the ability of any version to open any blend file, with 2.80 this is now broken. When some incompatibility was created with the addition of bmesh, we got an option of saving with Legacy Mesh Format so that older versions could open the file.

2.80 needs a similar option to save a backward compatible blend file.

The biggest issue is that no objects show up in the current scene. The objects and data are read from the blend file, they just don't get linked to the current scene. I expect this is related to the removal of the scene layers property.

Another issue is the screen layouts, having the loadUI option enabled makes a mess of the UI, I see the top info editor become a 3Dviewport, the outliner gets a blank display mode, the properties editor gets the tabs on the side (vertical space with horizontal icon list), various editor properties have new values that didn't exist in previous versions, eg. dopesheet mode, viewport material_mode. I also get a divider across the 3D viewport that follows the mouse.