Implement 2D layout of workspaces in overview

[mzur]

Implement 2D workspace display in the overview (see comment in workspacesView.js). What's missing is a correct implementation of the animation. Also, the viewport sometimes seems to move to the wrong location. Finally, the other workspaces are flickering/disappear sometimes.

I pushed my old branch gnome-40-overview again (from before the PR for actual GNOME 40 support). Maybe some of the changes there can be helpful here.