Add "play latest voice line" functionality / Quicksave

[drojf]

We had a report that you couldn't change language on the answer arcs in ADV mode. We don't plan to add support for Japanese language back in, but toggling language is useful for playing back the last voice line

Looking at 1c9d4ad, I think that we can make the TAB key play the last voice line/make a quicksave by just calling savegame 15 : loadgame 15

To add quicksave functionality, we just need some way to load the save in slot 15 (and make sure it isn't accidentally overwritten at other times during the game.

Note that this will also help us with this issue about autosaving during the eyecatch, as we can do the same savegame 15 : loadgame 15 during the eyecatch to make a quicksave.