Multi-Object Editing

This adds initial multi-object editing support.

- Selected objects are used when entering edit & pose modes.
- Selection & tools work on all objects however many tools need porting
  See: T54641 for remaining tasks.

Indentation will be done separately.

See patch: D3101

source/blender/blenkernel/intern/editmesh.c

@@ -89,10 +89,12 @@ BMEditMesh *BKE_editmesh_from_object(Object *ob)
 {
 	BLI_assert(ob->type == OB_MESH);
 	/* sanity check */
+#if 0 /* disable in mutlti-object edit. */
 #ifndef NDEBUG
 	if (((Mesh *)ob->data)->edit_btmesh) {
 		BLI_assert(((Mesh *)ob->data)->edit_btmesh->ob == ob);
 	}
+#endif
 #endif
 	return ((Mesh *)ob->data)->edit_btmesh;
 }