v0.6.0: Improved transient simulations y[n]

This release deals mainly with the "y[n]" tab for simulating impulse and transient response and a better integration of the FFT pop-up widget.

The internals of fixpoint and floating point simulations have been rewritten as a frame-based simulation. This is the base for allowing to interrupt a long running simulation and to load stimuli from file (both coming soon).

For details see CHANGELOG.md