Add advanced copy, cut and paste functionality

[erbridge]

We should mirror ST3's advanced copy/cut/paste functionality.

See #366.

[zoli]

I'll work on this.

[zoli]

Hi @erbridge I was reading the copy cut paste commands some questions occurred to me:

• What do you mean by TODO: Multiregion functionality will probably need to work differently.?
• Also TODO: Distinguish copying multiple regions from one region with multiple lines. how are these different?

[erbridge]

I just checked ST functionality again, and I'm wrong about how it works. I thought a multiline selection behaved differently from a multiregion one, but both get split up if pasting into the same number of regions as lines / copied regions.

You can safely remove both of those comments.

[zoli]

Ok I'm done with this for now, The paste command is not complete yet. If any one wants to finish paste command I've written a test for it here might be helpful.

[quarnster]

What's left to be done? Can we have a checklist or open up sub-issues please?

[zoli]

IIRC one of them is: If you copy when there is empty selection the clipboard will be whole line but on pasting whether you have a selection or not it will paste above the current active line.

[erbridge]

Migrated to limetext/backend#6.