[overlays] nested overlays having preventsScroll:true should not accumulate margins

[tlouisse]

Expected behavior

A clear and concise description of what you expected to happen.

Actual Behavior

Right now, all types of overlays add body margin. This should only happen for modal like overlays (preventsScroll: true).
This PR tackles that: #2331

However, it can still happen multiple overlays having preventsScroll: true are nested. Accumulation would still happen.

Solution

Interacting with the window/body should be handled on OverlaysManager (singleton) level. When the OverlaysManager detects another overlay is already handling body margin, it should keep margin as is.