Blender Git Loki

VSE: Automatic proxy building

Add `Proxy Setup` enum to user preferences with 3 choices: Manual, For Added strips and Automatic.
With `For Added strips` Only build proxies when adding movie strips.
With `Automatic` setting, proxies are built when preview size changes as well.

TODO:
- Decide what to do when workspace has multiple previews with different preview sizes. See `seq_get_preview_size()`
- Solution may be to change current design to allow only one size or just build multiple sizes.

Additional possible improvements:
- Cancel running job if preview size is changed while building proxies
- Use proxy of different size while building proxies for new size. it would be best to do after some refactoring, so in separate patch.
ref T85469

Maniphest Tasks: T85469

Differential Revision: https://developer.blender.org/D10363