Fix drag_value.edit_string unexpected reset

[tkkcc]

Make drag_value.edit_string None only during current focused widget update

Only two cases edit_string will be set to None (and reconstructed from value):

1. current widget gains focus.
2. current widget consumes up down key to change value.

Editing, Tabing and UpDown tested.

Close #2418
Close #2370

[tkkcc]

Still problematic. Editing DragValue then click another DragValue, the edit_string does not reset.

[tkkcc]

Fixed, add another reset on request_focus. Seems gained_focus is not satisfied this case.

[emilk]

@mwcampbell worked on this code recently - maybe you wanna take a look at this so it doesn't break AccessKit?

[mwcampbell]

I reviewed the code and tested it using only the keyboard and a screen reader, and it looks good to me.

[emilk]

Thanks for taking a look @mwcampbell! And thanks for the PR @tkkcc!