Added option `top_bar_alt_title`, which accepts an alternative title template to be displayed, and `top_bar_alt_title_place` that controls where it should be displayed. It can be either `below` to place it below the main one, or `toggle` to show it in place of the main title when user clicks the top bar or calls the `toggle-title` binding.
The `top_bar_title` now also accepts a custom top bar title template if user wants it to be different from the one defined in `mpv.conf`.
closes#402
8183163f2a