Refactor how custom level stats are stored, read, and written

There were a few problems with the old way of doing things:

(1) Level stats were an ad-hoc object. Basically, it's an object whose
attributes are stored in separate arrays, instead of being an actual
object with its attributes stored in one array.
(2) Level filenames with pipes in them could cause trouble. This is
because the filename attribute array was stored in the XML by being
separated by pipes.
(3) There was an arbitrary limit of only having 200 level stats, for
whatever reason.

To remedy this issue, I've made a new struct named CustomLevelStat that
is a proper object. The separate attribute arrays have been replaced
with a proper vector, which also doesn't have a size limit.

For compatibility with versions 2.2 and below, I've kept being able to
read the old format. This only happens if the new format doesn't exist.
However, I also WRITE the old format as well, in case you want to go
back to version 2.2 or below for whatever reason. It's slightly
wasteful to have both, but that way there's no risk of breaking
compatibility.

desktop_version/src/Game.h

@@ -76,6 +76,12 @@ struct MenuStackFrame
     enum Menu::MenuName name;
 };
 
+struct CustomLevelStat
+{
+    std::string name;
+    int score; //0 - not played, 1 - finished, 2 - all trinkets, 3 - finished, all trinkets
+};
+
 
 class Game
 {
@@ -363,9 +369,7 @@ public:
     void savecustomlevelstats();
     void updatecustomlevelstats(std::string clevel, int cscore);
 
-    std::string customlevelstats[200]; //string array containing level filenames
-    int customlevelscore[200];//0 - not played, 1 - finished, 2 - all trinkets, 3 - finished, all trinkets
-    int numcustomlevelstats;
+    std::vector<CustomLevelStat> customlevelstats;
     bool customlevelstatsloaded;