Fix that button presses don't set event as handled in AcceptDialog

[Sauermann]

resolve #77752
alternative to #77763

When clicking with the mouse on the OK-button of an AcceptDialog, the mouse event is not set to as handled.
This bug got uncovered by #77452, which removed a redundant is_inside_tree-check.
This is a more localized approach in comparison to #77763.

[KoBeWi]

This goes further though. The _ok_pressed() is just a callback for pressing the button. Shouldn't the event be handled by the button, which is the node that receives it?
(this also means that buttons never accepted the click events??)

[RedworkDE]

I did some more digging and the best way to avoid issues like this is probably to avoid removing the node in the event handler by deferring the hiding like the abort handler does by replacing set_visible(false); on line 124 with call_deferred(SNAME("hide"));.

The set_input_as_handled from this PR should probably be added anyways, unless we move that to Button (tho that should definitely be a different PR, as that probably breaks things)

[akien-mga]

Thanks!