Update DragValue when focus is lost

[YgorSouza]

Closes #2877.

[YgorSouza]

I came up with this not so elegant solution which consists of always drawing both the TextEdit and the Button, and making one of them invisible. This is both to ensure that the Ids don't change when we toggle between states, and to give the TextEdit a chance to update the value after it has lost focus.

As far as I can tell, there is only one case where it still doesn't work: when we click away from the DragValue directly into a TextEdit that was painted after it. This is due to #2142.

In other cases (pressing Tab or Shift + Tab, clicking on empty space or another type of widget, or clicking on a TextEdit painted before the current one) it updates correctly. However, this adds a penalty in performance, as there are always two widgets being added now. And I'm not sure how this affects accessibility, as I have no experience with that. I'm just leaving this PR for consideration in case no one comes up with a better solution.

[emilk]

I agree that the solution isn't so elegant :)

I came up with something else in #3776