[nemo-qml-plugin-contacts] Add detail accessor/mutator functions

For non-composite details, it is vital to know if the content has been
modified from an existing detail, or if it has been added independently.
Add a mechanism of extracting and re-inserting data that allows the
person to record modifications as such, maintaining the value of any
metadata in the detail record that was not directly changed in an edit
operation.
Merge pull request #62 from matthewvogt/detail-editing

[nemo-qml-plugin-contacts] Add detail accessor/mutator functions