Plot_window: Plot results from many simulations

Description of issue:

In some cases, you do transformations in the SFG, and have simulation results "before and after". Or you simulate with/without wordlength effects. In such cases, it would be handy to see those results in the same plot_window.

Potential Issues:

• The class and start_simulation_dialog can take take a list of sim results. [res1, res2, ...]
• The list can optinally include simulation names of the next res. ['name1', res1, 'name2', res2, ...]
  • The result is presented for the user as plot names like "name1.in1"
• Change function name from start_simulation_dialog to show_simulation_result.
• Add an "add_result" function to the class.
• Rebuild the GUI so it includes two tabs. One with todays time-domain plot, and one with a freqz result (see issue #195).
  • In this case, it may be a good idea to initially limit the time domain x axis to, say 0-100. This because you typically want perhaps 1000 points in the frequency domain, and this scale often makes the time-domain plot hard to read.
• Today, the user just type simulation.show(). We can tell the user to run show_simulation_result(['name1',res1, ...])

Edit: The list above is updated and incorporates suggestions from the comments.