Support multiple scenes in a single file

Is your feature request related to a problem? Please describe.

There are cases where a stash user would want to view scenes that may exist in a single file separately without re-encoding the source file into separate files so they can be viewed as separate scenes within stash

Describe the solution you'd like

The solution has been discussed a few times and usually revolves around utilizing a similar system to Markers to determine the start and end points of a "sub-scene" within a file that has multiple scenes while leaving the source file as is

Describe alternatives you've considered

re-encoding the source file into separate files, this is undesirable as it changes the fingerprints of the file and goes against a core feature of stash where the source files remain untouched

related issues: #260 #779