[ProbeStore] Expose size
[ProbeStore] Assert pointer

We can assert QSharedPointers just like regular pointers, so do so.
[ProbeStore] Emit signals on data change

Emit newStation/newSSID signals so we can change the display when we get
new data.
[ProbeStore] Add retrieval function (for the key)

Add the ability to retrieve MacAddresses from the ProbeStore. It'll be
used to display the list of stations we have information about.
[ProbesModel] Add a tree widget (and model) for displaying the detected stations