DNA: clear / remove deprecated flags

- Clear deprecated flags for ID's:
  Scene, Sequence, World, Object & Mesh.
- Clear deprecated flags for Spaces: outliner, 3D view & image.
- Remove unused `Mesh.drawflag`
- Remove unused `USER_ALLWINCODECS`, `USER_MMB_PASTE`.
- Remove `V3D_SOLID_TEX` & `V3D_ZBUF_SELECT` - used in a few areas.
- Flip `Object.empty_image_visibility_flag`
  (avoids do-version on each new flag)
- Rename 'Backside' -> 'Back' in context of drawing - showing 'Back'
  makes sense.

release/scripts/startup/bl_ui/properties_material.py

@@ -229,7 +229,7 @@ class EEVEE_MATERIAL_PT_settings(MaterialButtonsPanel, Panel):
             row.prop(mat, "alpha_threshold")
 
         if mat.blend_method not in {'OPAQUE', 'CLIP', 'HASHED'}:
-            layout.prop(mat, "show_transparent_backside")
+            layout.prop(mat, "show_transparent_back")
 
         layout.prop(mat, "use_screen_refraction")
         layout.prop(mat, "refraction_depth")