Camera tracking: switch dopesheet information to lazy calculation

All operators which changes tracking data now just tags dopsheet as outdated,
actual re-calculaiton of happens only when this information is actually needed
(like on dopesheet draw).

This makes things a bit faster when there's no dopesheet visible in current
screen and also makes it much easier to update dopesheet using dependency
graph.

Also renamed dopesheet_sort_order to dopesheet_sort_method in rna and internal
stuff which makes much more sense and also correlated with naming in
file browser.

release/scripts/startup/bl_ui/space_clip.py

@@ -86,7 +86,7 @@ class CLIP_HT_header(Header):
 
             if sc.view == 'DOPESHEET':
                 layout.label(text="Sort by:")
-                layout.prop(sc, "dopesheet_sort_order", text="")
+                layout.prop(sc, "dopesheet_sort_method", text="")
                 layout.prop(sc, "invert_dopesheet_sort", text="Invert")
 
         layout.template_running_jobs()