History view: allow older waveforms to be deleted without waiting for them to fall off the end of the buffer

[mubes]

It would enhance the usability of the history panel if we could;

a) Delete uninteresting historic sample sets
b) Provide a short text label to accompany a specific sample set
c) 'pin' a sample set so it does not get deleted
d) (Now I'm getting silly) Allow a set of historic samples to be 'ghosted' onto the current sample buffer

The combination of these features would allow the history panel to be used for A/B comparisons etc, and really enhance its utility.

[azonenberg]

a) good suggestion

b/c) duplicate of #283

d) duplicate of #217

[mubes]

Hmm....mebbee it's time I read through all the outstanding enhancement requests.....