Code review comment for lp:~kdub/mir/demo-stream-titlebars

I've made a few attempts on this topic over the years. Although I feel treating the titlebar as the only type of decoration is quite wrong, simultaneously ignoring the past and not looking to the future.

Please read:
https://bugs.launchpad.net/mir/+bug/1569836/comments/1

Also consider how you would support a theme with:
  * Window borders around the sides
  * No titlebar
  * Titlebar on the side
  * Titlebar non-rectangular, or not the same length as the window
  * Titlebar on the bottom
  * A titlebar that is hidden and only pops up sometimes, maybe with floating buttons.

There is prior art for all of these. The only commonality really is that the client area is usually (but not always) rectangular. And decorations may be arbitrary.

I hesitate to disapprove, because the problem is also pre-existing. But we should be thinking about more generic solutions than this.