Save showtargets to main game save files

This fixes an oversight that could lead to confusion by the player.

showtargets is the variable that shows all unexplored teleporters on the
map as a question mark, so players know where to head to to make
progress. However, it previously was not directly saved to the main game
file. Instead, it would be set to true if flag 12 was turned on in the
save file.

How well does flag 12 correlate with showtargets?

Well, the script that turns on showtargets (bigopenworld and
bigopenworldskip) doesn't turn it on. Neither does completing Space
Station 1.

This flag is only turned on when the player activates Violet's activity
zone for the first time.

Therefore, it's entirely possible that a new player could complete Space
Station 1, then save their game, and come back to resume playing later.
When they do come back, the question marks that Violet told them about
won't show up on the minimap, and they'll be confused. They may not know
where to go.

And it is completely unintuitive for them to know that in order to get
the question marks to show up again, they have to not only talk to
Violet, but then save the game again, and reload the save. Especially
since the question marks only show up after you reload the save, and not
when you talk to Violet (because flag 12 is only a proxy for
showtargets, not the actual variable itself).

So what's the solution? Just save showtargets to the save file directly.

desktop_version/src/Game.cpp

@@ -4971,6 +4971,10 @@ void Game::readmaingamesave(tinyxml2::XMLDocument& doc)
                 music.play(song);
             }
         }
+        else if (SDL_strcmp(pKey, "showtargets") == 0)
+        {
+            map.showtargets = help.Int(pText);
+        }
 
     }
 
@@ -5495,6 +5499,8 @@ std::string Game::writemaingamesave(tinyxml2::XMLDocument& doc)
         xml::update_tag(msgs, "currentsong", music.currentsong);
     }
 
+    xml::update_tag(msgs, "showtargets", (int) map.showtargets);
+
     xml::update_tag(msgs, "teleportscript", teleportscript.c_str());
     xml::update_tag(msgs, "companion", companion);