Unload zips before loading zips

This fixes a minor issue where if you had a zip in the levels list, but
then removed it, it would still show up in the levels list after
reloading it (if you also had a .vvvvvv file named the same as in the
zip) even though it shouldn't.

Thankfully, this didn't lead to a memory leak or duplicate zip mounts or
anything like that, because PhysFS ignores mounting a zip if it's
already mounted.

This also didn't result in a level entry from a zip persisting after
removal after reloading the levels list, because the entry would be gone
due to the .vvvvvv file not being found.

desktop_version/src/CustomLevels.cpp

@@ -251,6 +251,23 @@ static void levelMetaDataCallback(const char* filename)
     }
 }
 
+static void unloadZips(void)
+{
+    char** list = PHYSFS_getSearchPath();
+    if (list == NULL)
+    {
+        return;
+    }
+    for (char** path = list; *path != NULL; path++)
+    {
+        if (SDL_strncmp(*path, "levels/", 7) == 0 && endsWith(*path, ".zip"))
+        {
+            PHYSFS_unmount(*path);
+        }
+    }
+    PHYSFS_freeList(list);
+}
+
 void customlevelclass::getDirectoryData(void)
 {
 
@@ -258,6 +275,8 @@ void customlevelclass::getDirectoryData(void)
 
     FILESYSTEM_clearLevelDirError();
 
+    unloadZips();
+
     loadZips();
 
     FILESYSTEM_enumerateLevelDirFileNames(levelMetaDataCallback);