Blender Git Loki

Refactor of proxies build operators

Split proxy build operator into three parts:
- Prepare context (IMB_anim_index_rebuild_context) which prepares all
needed data and stores it in an anonymous structure used by specific
builder lately.
- Build proxies/timecodes into temporary files (IMB_anim_index_rebuild)
This function will build all selected proxies/timecodes into a temporary
files so old proxies will be still available during building.
- Finish building proxies (IMB_anim_index_rebuild_finish) which copies
temporary files over old proxies filed and releases all resources used
by a context.

Context creation and finishing building happens in a main thread so
it's easy and safe to close all opened handles of proxies files and
refresh cache after rebuilding is finished.

This should finally fix #30315: Temporary proxy files are not erased and old proxys are not updated if the proxy is built more then once (windows)