Changed semantic of recently added start_frame

Now it's indicates at which scene frame number movie clip starts playing back.
This this setting is still belongs to clip datavlock and used by all users of
clip such as movie compositor nodes, constraints and so.

After long discussion and thoughts about this it was decided that this would
match image's current behavior (which initially seen a bit crappy), but that's
actually allows:

- Keep semantics of start frame in image and clip datablocks in sync
- Allows to support features like support of loading image sequences
  with crappy numbers in suffix which doesn't fit long int.
- Allows to eliminate extra boolean checkbox to control such kind of offset.

Hopefully from pipeline POV it wouldn't hurt because idea of having this things
implemented in original way was  working only if sequence before processing
started naming form 001.

release/scripts/startup/bl_ui/space_clip.py

@@ -960,9 +960,7 @@ class CLIP_PT_footage(CLIP_PT_clip_view_panel, Panel):
 
         col = layout.column()
         col.template_movieclip(sc, "clip", compact=True)
-        col.prop(clip, "use_custom_start_frame")
-        if clip.use_custom_start_frame:
-            col.prop(clip, "start_frame")
+        col.prop(clip, "start_frame")
 
 
 class CLIP_PT_tools_clip(CLIP_PT_clip_view_panel, Panel):