[nemo-qml-plugin-contacts] Allow contacts to be filtered by required property

Where a required property is specified, only contacts which possess one
or more details of the required type are reported.
[nemo-qml-plugin-contacts] Allow name groups to be filtered by required property
Merge pull request #26 from matthewvogt/filter

[nemo-qml-plugin-contacts] Allow contacts to be filtered by required property