Refactor explored rooms to use setters and getters

This makes it easier to add bounds checks to all accesses of
map.explored. Also, all manually-written existing bounds checks have
been removed, because they're going to go into the new getters and
setters.

The getter is mapclass::isexplored() and the setter is
mapclass::setexplored().

desktop_version/src/Map.h

@@ -92,6 +92,9 @@ public:
     bool explored[20 * 20];
     int vmult[30];
 
+    bool isexplored(const int rx, const int ry);
+    void setexplored(const int rx, const int ry, const bool status);
+
     int background;
     int rcol;
     int tileset;