Camera tracking: add camera to follow track and object solver constraint

Object used to be parented to active camera which isn't very convenient when
working with witness cameras.

Now parent camera can be specified in constraint (if it's not specified, active camera is used)

release/scripts/startup/bl_ui/properties_object_constraint.py

@@ -776,6 +776,8 @@ class ConstraintButtonsPanel():
             layout.prop_search(con, "object", clip.tracking, "objects", icon='OBJECT_DATA')
             layout.prop_search(con, "track", clip.tracking, "tracks", icon='ANIMATION_DATA')
 
+        layout.prop(con, "camera")
+
         layout.operator("clip.constraint_to_fcurve")
 
     def CAMERA_SOLVER(self, context, layout, con):
@@ -798,6 +800,8 @@ class ConstraintButtonsPanel():
         if clip:
             layout.prop_search(con, "object", clip.tracking, "objects", icon='OBJECT_DATA')
 
+        layout.prop(con, "camera")
+
         row = layout.row()
         row.operator("constraint.objectsolver_set_inverse")
         row.operator("constraint.objectsolver_clear_inverse")