Added Spacer to be used on ReMenuCategory / ReCategoryPage

[MintLily]

Usage:

private ReMenuCategory Menu;
private ReCategoryPage Page;

private void Build() {
    Page = Menu.AddCategoryPage("Name", "ToolTip", SpriteIcon);
    var m = Page.AddCategory("Title Header");
    m.AddSpacer(); // previously, this was not accessible
}