Previously, the importer assumed scenes had fields that sources did for
audio, namely volume, sync, mute and monitoring type. SL scenes do not
have these fields. This resulted in mistakes in importing, such as
imported scenes having a volume of 0.0.
This change gets rid of the non-existent fields, and explicitly sets the
volume of the scenes to 1.0 as a precaution.
The previous fix for repeated names accidentally meant that if a scene
item in a scene was meant to show another scene that was lower in the
list, it would fail to create the item, and wouldn't make it into the
imported collection.
This fix makes sure that scenes as sources still works properly in that
situation.
This replaces the previous Open File dialog for importing collections
with a window for importing many collections at once, based on the remux
window, along with support for importing from OBS Classic, XSplit
Broadcaster and from Streamlabs' fork. This also translates sources
between OSes that Studio supports.
