Blender Git Loki

VSE: Refactor VSE strip loading code

Isolate RNA and operator logic from functions that create strips.
- Operator specific code was removed from `SeqLoadInfo` structure and
`SEQ_add_*` functions.
- Strip loading code was removed from RNA and operator functions.
- `SEQ_add_*` API was unified to work on `SeqLoadData` struct.
Only exception is image strip, which require files to be loaded
separately to strip creation itself. This is not ideal, but I think
it's acceptable.
- Some functions and variables were refactored so the code reads
better.

There are minor functional changes (coincidental bugfixes):
- Operator errors are reported per-strip. Previously they were not
reported at all?
- `new_sound()` RNA API function now create sound with length of 1
if source file does not exist. Previously it created strip with
length of 0.
- Replace selection operator property wasn't working correctly.
Fixed in this patch.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D9760