placement: make new window cascading optional
Cascading behaviour was introduced with !3229 (merged)
This change makes the cascading behaviour optional, which:
- allows users to disable it if preferred
- fixes the issue that without this option the window placement
policies no longer do explicitly what they say they do (e.g. setting
the placement policy to
Center, and having windows sometimes not opening centered without the user knowing why)
| Before | After |
|---|---|
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