Resources: Perform legacy savegame conversion in the background

Improved conversion mechanism using a background TaskPool. Each
savegame conversion is handled using a separate concurrent task,
scheduled via Doomsday Script in bootstrap.de

A resource system ConvertSavegameTask delegates the conversion to
a Doomsday savegame converter plugin.

The Savegame Converter plugin now executes Savegame Tool using a
subprocess which blocks until conversion is completed.

ResourceSystem observes the conversion task pool, repopulating the
file system automatically whenever the pool is empty.

Todo: There is presently an active plugin id conflict, caused by
the concurrently running conversion tasks, which results in game
registration failures during startup.