Add and use Game::save_exists()

This is simply a shorthand for telesummary != "" || quicksummary != "",
to make it easier and less error-prone to negate. This improves
readability.

desktop_version/src/Game.h

@@ -328,6 +328,7 @@ public:
     std::string activity_lastprompt;
 
     std::string telesummary, quicksummary, customquicksummary;
+    bool save_exists();
 
     bool backgroundtext;