[nemo-qml-plugin-configuration] Add a ConfigurationGroup type.

This provides a convenient API for getting and setting multiple
properties under a common key path.

Properties declared in a ConfigurationGroup are populated with values from the store, and values subsequently assigned to the the property are written back to the store.  Groups can also be nested to access hierarchical structures.

ConfigurationGroup {
    path: "/path/to/my/settings"
    property int myIntegerProperty: 5
    property string myStringProperty: "hello world"

    ConfigurationGroup {
        path: "nested"
        property real myNestedProperty: 0.3
    }
}