This change makes it so any strings in a scene collection that start
with "./" are checked as being a relative path. As long as the resulting
absolute path is contained within the same directory as the collection
being imported, it will be replaced with an absolute path.
This allows scene collections to be effectively "packaged" with assets,
so long as the assets are contained somewhere within the folder the
collection is being imported from.
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.