kcm/kwinrules: Update values on text edit

Previously the text values were updated to the model after
`onEditingFinished()`, that is after losing focus, to prevent
erroneus updates.

This was making also the `needsSaving` signal fire only after
a focus change, which is not consistent with the behavior of
other KCMs.

Use `onTextEdit()` instead, so the model is updated as the user
types.

BUG: 431211