Small cleanups

* Instantiate the FindDialog on demand only.
* When submitting a contact and there was an error, return early to
avoid adding the incomplete contact.
* In the FindDialog's findClicked slot call accept() or reject() on the
dialog, to propagate the acceptance to the caller in
AddressBook::findContact.
* Renamed FindDialog::getFindText to FindDialog::findText()

Reviewed-by: Kavindra