Blender Git Loki

VSE: Improve motion-picture workflow

This commit resolves problem introduced in e1665c3d3190 - it was
difficult to import media at their original resolution.
This is done by using original resolution as reference for scale.

All crop and strip transform values and their animation is converted
form old files.

To make both workflows easy to use, sequencer tool settings have been
created with preset for preffered scaling method. This setting is in
sequencer timeline header and add image or movie strip operator
properties.

Two new operators have been added:
`sequencer.strip_transform_fit` operator with 3 options: Scale To Fit,
Scale to Fill and Stretch To Fill.
Operator can fail if strip image or video is not loaded currently, this
case should be either sanitized or data loaded on demand.

`sequencer.strip_transform_clear` operator with 4 options:
Clear position, scale, rotation and all (previous 3 options combined).

Reviewed By: sergey, fsiddi

Differential Revision: https://developer.blender.org/D9582