Decouple saving in savelogic via internal signal

[drogenlied]

Migration: trac#195.

Not feasible right now, we need an interface to specify how we can close plots and how the data to be saved is guarded against changes while the save logic is working on the data in the module wanting to save data.

[Neverhorst]

One could simply employ the same "handshake" functionality as in other logic modules. Tested a version of pulsed_measurement_logic that is connecting a signal to the save_data method in save_logic. If the data is passed through the signal emit, the measurement_logic can change it in the meantime and it will not mess up the passed data. If the save_logic would then emit something like a "sigSaveComplete" signal, the logic/GUI gets updated and knows that the save process has completed. In the meantime nothing hangs/freezes.