[nemo-qml-plugin-contacts] Emit change signals properly

Emit changed signals explicitly rather than by comparison with an
empty QContact object, otherwise the following scenario can prevent
signal emission:
 - internal contact has a detail containing a value which is different
   to the default-constructed value
 - contact is updated, changing the value to the default-constructed
   value
 - the new value is compared with the same value retrieved from the
   empty contact, which is the defaul-constructed value
 - no change is detected, and thus no signal is emitted
[nemo-qml-plugin-contacts] Do not calculate display label lazily

Lazy calculation can result in a change signal being emitted from
an accessor function, which can be wrongly detected by the QML engine
as a binding loop.