Allow levels to select a font via XML, show correct font in levels list

There still needs to be a menu for selecting a font, but it can now be
loaded and saved correctly in the XML!

desktop_version/src/CustomLevels.h

@@ -63,6 +63,10 @@ struct LevelMetaData
      * was stored in this struct (for the levels list) */
     bool title_is_gettext;
     bool creator_is_gettext;
+
+    /* This is for the metadata in the levels list,
+     * so it will only be a main font (no custom ones). */
+    uint8_t level_main_font_idx;
 };
 
 struct CliPlaytestArgs
@@ -158,6 +162,8 @@ public:
     int levmusic;
     int mapwidth, mapheight; //Actual width and height of stage
 
+    std::string level_font_name;
+
     int version;
 
     SDL_Color getonewaycol(int rx, int ry);