Better tab relabelling

[davep]

Implements a better method of changing the label of a Tab; fixes #3901.

This is a fairly small change, which turns Tab.label into a safe way of setting a new label for a Tab. So the question in the original issue would end up being answered with this:

self.query_one(TabbedContent).get_tab("one").label = "Renamed!"

It also quietly supports going via Tab.update too but doesn't advertise it as the approach to take.

The PR also, of course, addresses the issue of the underline not redrawing correctly.

Screen.Recording.2024-01-08.at.15.52.31.mov

[rodrigogiraoserrao]

LGTM 🚀🐍