VSE: Python API, allow creation of VSE Movie strips with missing file

It was already possible to create Sound and Image strips that reference
non-existing files. Now it's also possible to create Movie strips
referencing missing files via the Python API call
`Sequences.new_movie()`. In this case, the duration of the strip will be
set to 1 frame.

Note that this commit does not change anything in the user interface.

The Python API of the `MovieStrip` class is extended with a function
`reload_if_needed()`. This function only performs disk I/O if the movie
strip cannot produce frames, that is either when its filepath points to
a non-existing file, or when the video sequence editor has not been
shown yet (for example because it is in an inactive workspace).

This allows for the following:

```
import bpy

scene = bpy.context.scene
vse = scene.sequence_editor_create()

filepath = bpy.path.abspath('//demo.mkv')
strip = vse.sequences.new_movie("movie", filepath,
    channel=2,
    frame_start=47,
    file_must_exist=False)
strip.frame_final_end = 327
```

This will create a new movie strip, even when `demo.mkv` does not exist.

Once `demo.mkv` has appeared at the expected location, either
`strip.reload_if_needed()` or `strip.filepath = strip.filepath` will
load it.

Differential Revision: https://developer.blender.org/D8257

Reviewed By: Sergey, ISS

source/blender/blenkernel/BKE_sequencer.h

@@ -285,6 +285,11 @@ void BKE_sequence_reload_new_file(struct Main *bmain,
                                   struct Scene *scene,
                                   struct Sequence *seq,
                                   const bool lock_range);
+void BKE_sequence_movie_reload_if_needed(struct Main *bmain,
+                                         struct Scene *scene,
+                                         struct Sequence *seq,
+                                         bool *r_was_reloaded,
+                                         bool *r_can_produce_frames);
 int BKE_sequencer_evaluate_frame(struct Scene *scene, int cfra);
 int BKE_sequencer_get_shown_sequences(struct ListBase *seqbasep,
                                       int cfra,