Center vertically/horizontally PopupPlasmaWindows when they do not fit boundaries

Niccolò Venerandirequested to merge
work/niccolove/500390 into master

Previously, whenever a PopupPlasmaWindow would not fit the given boundary, it would be left-aligned to the parent element. This was not intuitive and would be a significant change to when the popup could fit the boundary. For a better transition, we vertically/horizontally center align the popup to the given boundary when it's not possible to fit completely within it.

BUG:500390

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