workspace-indicator: Use custom layout manager for thumbnails

The current code positions window previews explicitly using a fixed
layout manager. For that it relies on a valid parent allocation,
which is error-prone and frequently results in warnings.

Address this by moving the positioning code into a custom layout
manager, and only update the visibility from the window preview.

https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/issues/260