this simplifies error handling statements all over the ui code. Before
we had to write:
if err := foo.Render(target); err != nil {
return err
}
which simplifies now to foo.Render(target)
the uiManager now handles every element of the ui, so we don't need to
render elements manually in game_controls. Now we can also use
widget_groups to simplify handling the opening/closing of the panel.
