windowPreview: Cache the boundingBox of the layout manager

windowPreview: Cache the boundingBox of the layout manager

Accessing GObject properties from JS has proven to be quite slow because
of the JS->C->JS roundtrip involved. With the WindowPreview this
actually has an impact since we're accessing those properties very often
while creating new layouts.

So cache the boundingBox and the windowCenter properties of the
WindowPreview using a this._cachedBoundingBox JS object. This might
speed up opening the overview with lots of open windows significantly.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1617>