BSP Entities lump writes "classname" and "model" keys twice on write

[SamVanheer]

When a BSP file's entities lump is written it writes the classname and model keys twice. This is because it writes both all keyvalues and these keyvalues explicitly:

sledge-formats/Sledge.Formats.Bsp/Lumps/Entities.cs

Lines 157 to 170 in 72abe60

	if (entity.Model > 0)
	{
	sb.Append($"\"model\" \"*{entity.Model}\"\n");
	}
	foreach (var kv in entity.KeyValues.Where(x => x.Key?.Length > 0 && x.Value?.Length > 0))
	{
	sb.Append($"\"{kv.Key}\" \"{kv.Value}\"\n");
	}
	if (entity.ClassName?.Length > 0)
	{
	sb.Append($"\"classname\" \"{entity.ClassName}\"\n");
	}

Both keyvalues are added to the list of keyvalues on load:

sledge-formats/Sledge.Formats.Bsp/Lumps/Entities.cs

Lines 124 to 136 in 72abe60

	void SetKeyValue(Entity e, string k, string v)
	{
	e.KeyValues[k] = v;
	switch (k)
	{
	case "classname":
	e.ClassName = v;
	break;
	case "model":
	if (int.TryParse(v.Substring(1), out var m)) e.Model = m;
	break;
	}
	}

It should use one or the other to save these keyvalues. It might be better to remove the explicit keyvalue properties for classname and model and rely solely on the keyvalues dictionary, since users will likely have their own wrapper around this data.