Add being able to delete a custom level quicksave

Custom level quicksaves are NOT affected by the clear data menu, so the
player should be able to delete quicksaves this way. The quicksave
confirmation menu now has an extra option to delete the save (and that
option also has its own confirmation menu before deleting).

desktop_version/src/Game.h

@@ -32,6 +32,7 @@ namespace Menu
         playerworlds,
         levellist,
         quickloadlevel,
+        deletequicklevel,
         youwannaquit,
         errornostart,
         errorsavingsettings,
@@ -197,6 +198,8 @@ public:
     void customloadquick(std::string savfile);
     void loadquick(void);
 
+    void customdeletequick(const std::string& file);
+
     void loadsummary(void);
 
     void readmaingamesave(tinyxml2::XMLDocument& doc);