Allow overriding SpinBox value on focus_exited

[larspet]

Fixes #100836

When setting SpinBox's value the text is only updated on NOTIFICATION_DRAW. If the value is changed on the same frame as the focus is lost, this means the old text will be submitted instead. This PR thus checks if the value has changed before submitting the text.

With this PR the MRP will:

• Correctly override the value on focus_exited.
• Not call _val_changed() when using set_value_no_signal().
• Only call _val_changed() once when directly setting value.

[KoBeWi]

I gave it some testing and didn't spot any problems.

[akien-mga]

Thanks!