Object tracking: object with object solver constraint is now parented to scene's camera

Made Object Solver operator parent object to scene's camera. Behavior is pretty much
familiar to Child Of constraint -- it stores inverted transformation matrix which gives
constant offset in parent's space.
Current files would open incorrect, to make object aligned well again, just press
"Set Inverse" button in Object Solver constraint.
Fixed orientation operators so now they should work in all cases.
Also changed behavior of Set Origin operator which now sets origin to the median
point of all selected tracks/

release/scripts/startup/bl_ui/properties_object_constraint.py

@@ -784,6 +784,10 @@ class ConstraintButtonsPanel():
 
         layout.prop(con, "object")
 
+        row = layout.row()
+        row.operator("constraint.objectsolver_set_inverse")
+        row.operator("constraint.objectsolver_clear_inverse")
+
         layout.operator("clip.constraint_to_fcurve")
 
     def SCRIPT(self, context, layout, con):