#285 PGN export does not escape quotes (and backslashes?) in header tag strings

[offroadtrip]

Exporting a filter to PGN results in unescaped quotation marks in tag strings (don't ask me why organizers would put quotes in their event name); this results in a PGN that doesn't import correctly. I believe the issue is in game.cpp at around line 2286; tacking on a helper function to insert them fixed it for my use-case.