view: implement `cascade` placement policy

view: implement `cascade` placement policy

Adds following settings:
<placement>
  <policy>cascade</policy>
  <cascadeOffset x="40" y="30" />
</placement>

"Cascade" policy places a new window at the center of the screen like
"center" policy, but possibly shifts its position to bottom-right so the
new window doesn't cover existing windows.

The algorithm is copied from KWin's implementation:
https://github.com/KDE/kwin/blob/df9f8f8346b5b7645578e37365dabb1a7b02ca5a/src/placement.cpp#L589

Also added some helper functions to manipulate `wlr_box`.