Ability to save recorded plot data

[jpoeppel]

Sometimes we might want to save the received data, e.g. in order to create different plots from it later manually.
While the producing code could be adapted to store the data, it would be easier if we would just serialize the data we have stored in each channel anyways.

We will need to decide on a format (e.g. everything in one file vs one file for each channel) as well as the actual UI. Maybe an entry in the context menu, which opens a popup where one can specify which channels he wants to store.