Fix window behavior when default button is present

* Don't intercept Enter key press in a window when there is default
button and another BControl focused.
* Current behavior was basically breaking every window using default
button. Even if there was a button focused (using Tab key), when Enter
was hit, default button handled the message, therefore instead of
i.e. 'Revert' 'Apply' was pressed.
* Fixes #10792.

Signed-off-by: Jessica Hamilton <jessica.l.hamilton@gmail.com>