Messages shouldn't be suppressed in programmatic changes

[rodrigogiraoserrao]

TL;DR: We should not prevent a message from being posted just because the state of the widget was changed programmatically, and even though the programmer already knows the state changed.

Immediate effect: TabbedContent will now post TabActivated when the reactive attribute active is assigned to.

Possible further considerations:

• Add a message Completed to the ProgressBar widget.
• Add a message Started to the ProgressBar widget.
• ...?

Thesis:
Most widgets will post appropriate messages when their state changes, even if through programmatic means.
E.g.,

• DataTable will post CellHighlighted if we move the cursor with .move_cursor or by assigning directly to cursor_coordinate;
• Checkbox and RadioButton will post Changed if we toggle them with .toggle or by assigning directly to value; and
• OptionList will post OptionHighlighted if we assign to the reactive highlighted.

I think all programmatic changes should still post the appropriate messages that reflect the changes that happened.

Even if a programmer does changes like this programmatically, they are sensible in assuming that the respective events will be triggered.
This lets them decouple two things:

• the logic that will change some state; and
• the logic that handles changes to the state.

If a programmer does not want the message, they can explicitly opt-out with the context manager prevents, which is why the context manager was created in the first place.

To consider if this is accepted.

1. For the issue with TabbedContent: the usage of prevent in the TabbedContent comes from Prevent reactive-watcher loop in Tabs / TabbedContent. #2305 and Revert "Prevent reactive-watcher loop in Tabs / TabbedContent." #2322, so take those two PRs into account when fixing this.
2. Consider if there are other events missing that a user might want to be notified about.

[rodrigogiraoserrao]

Settled with #3417.

We post all messages.

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