Recognize zipped savegame when no Zlib was compiled
Every now and then we get reports on problems with transferring savegames from platform which haz zLib support to platform which does not or has a build which has it disabled for some reason.
Instead of stating that save is in invalid format we should (a) in scumm engine show that savegame "GZip compressed (uncompress it manually)". And SaveFileManager should report it and appropriate message should be shown for all other engines.
If there will be some standard code, then, perhaps some standard wrapper function should be added to OSystem, or typical code snippet added to Engines HOWTO on our Wiki.
Ticket imported from: #1615949. Ticket imported from: feature-requests/343.