Add filament retract and unretract user variables for use in base and helper macros

[tehniemer]

Add user configurable variable for filament retract in end and cancel print macros.

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[tehniemer]

I think this is still a valid addition.

[Frix-x]

Thanks for the PR!

I want to merge this. But as stated in #658 this need to be linked to the loading during a purge at the beginning of a print. Indeed, what you retract at the end of a print should be inserted again at the beginning of the next one (at least it was designed like that).

And this can be probably linked to #645 and #641 at the same time.

Can you please complete the PR with this additional need?

[wight554]

In my testing I noticed that it's better to push a little bit more than retracted, like 10-20% more
I retract 30 and push 35, otherwise half of my purge line is somewhat empty. However would be better if anyone else shares their experience

[tehniemer]

I just added an unretract to the prime line macro equal to the retract in the cancel and end print macros. Please let me know if that's good enough or if you would like an additional user variable for the extra push as mentioned by @wight554

[Frix-x]

Yes, that's right, I think having a percentage factor combined to this could be a good idea.
Like by default 15% more: it doesn't cost much to do this and should be pretty harmless.

[tehniemer]

OK, I've added #641 and #645 into this one and created a second variable for the nozzle prime.

[wight554]

The way I see it is letting user set unretract extra length similarly to slicer. Feels better UX wise. Not very critical though

[Frix-x]

I think we are near a merge. Just a few changes on my side, but nothing very problematic :)

[tehniemer]

I'm good with both of those.

[Frix-x]

Ok I'm good with it now and will merge! Thank you :)