Add Quantize button to Piano Roll (WIP)

[grejppi]

WORK IN PROGRESS

Inspired by a conversation on #lmms:

14:03 < klopsi-u3> is there a way to force an already recorded track to a lower 
                   quantisation?
...
14:42 < grejppi> klopsi-u3: none in the interface
14:42 < grejppi> that would be a useful feature though
14:44 < klopsi-u3> ok
14:45  * grejppi might hack that in
14:45 < klopsi-u3> :)
14:45 < grejppi> or at least try
14:46 < grejppi> I've already been sidetracked by so many things today c:
14:50 < klopsi-u3> i think there should be a seperate button for 'quantise 
                   piano roll'
14:50 < klopsi-u3> iow, not automatically shift quantisation if the 
                   quantisation dropdown is changed
14:51 < grejppi> yeah
14:51 < klopsi-u3> maybe even only quantise selected notes

There is now a new button in the Piano Roll editor toolbar:

If there is a selection, it will quantize the selected area to the current quantization level (1/4 in these screenshots for clarity):

Without a selection the entire pattern is quantized.

Known issues:

• As you can see from the last screenshot, there are still cases where some notes are left as-is. I will investigate this.
• This has absolutely not been tested yet. At least it builds for now.

[BaraMGB]

There should be a difference between length and position quantization.

[grejppi]

@BaraMGB that would be a matter of changing the behaviour of Note::quantizeLength().

[zonkmachine]

Cool! Tested it and it seem to work just fine.

[zonkmachine]

Swing?

[grejppi]

@zonkmachine what happens in that screenshot?

[zonkmachine]

Yes, that wasn't all too clear.
I'm trying to show that from implementing quantisation to adding swing/groove shouldn't be that much work. I haven't dug any deeper into the various versions of swing but an Issue for it with links to articles is here -> #1091

[grejppi]

@LMMS/developers I don't generally like merging my own pull requests, but unless there are any objections I will do that in 24 hours. This does not seem to break existing functionality 👍

[tresf]

👍