workspaceThumbnail: Consider porthole offsets

Since commit 9980c806, the porthole position is ignored. As a result, previews are only shown if the primary monitor is located at (0, 0).

To fix this, we either need to propagate the porthole to every thumbnail, use a custom layout manager that applies an offset to all children, or add an intermediate actor that offsets the contents.

The last option is the simplest and doesn't require calls into JS on every allocation, so pick that one.

Fixes #3781 (closed)