SCI: Implement the file operations needed for the save dialog in Phantasmagoria

Phantasmagoria's scripts keep polling for the existence of the savegame
index file and request to read and write it using the same parameters
when opening it. The index file is closed and reopened for every save
slot, which is slow and can be much slower on non-desktop devices.
Also, the game scripts request seeking in writable streams and request
to expand the existing index file.

To provide this functionality and to reduce constant slow file opening
and closing, this virtual class has been introduced

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