clutter/text: Queue a relayout after text updates

This ensures the actor is actually reallocated and not just repainted after the text changes and fixes the issue that the old, remaining allocation is smaller than the width of the new text.

One situation where this happens right now is with the new App menu of the shell: When the workspace is changed (and with it the focus-window) and the old label was smaller than the new one, the new label will be truncated to the old width. This does only seem to happen if the previously focused app has a window open on both workspaces though.