In the original gimp-2-10 branch where this feature was originally
worked on, we had to keep gimp_checks_get_shades(). But on `master`
where we will reset and break API, let's get rid of this function which
should not be used anymore.

I cleaned many remaining places where the concept of linked item still
survived.
On loading an XCF file with linked items, we are now going to create a
named sets for the linked items, allowing people to easily select these
back if the relation was still needed.

We don't remove gimp_item_get_linked() yet and in particular, we don't
save stored items into XCF files. This will come in an upcoming change.

luz paz authored Nov 05, 2020 and Jehan committed Nov 19, 2020

Found via:
```
codespell -q 3 -S ./ChangeLog*,*.po,./.git,./NEWS* -L als,ang,ba,chello,daa,doubleclick,foto,hist,iff,inport,klass,mut,nd,ower,paeth,params,pard,pevent,sinc,thru,tim,uint
```

Let's start up some porting guide for plug-in developers. One of the
first things to do is to get rid of any deprecated functions from 2.10
or older versions. In the current stable, these would "just" output
warnings, but in the dev version, all deprecated functions got removed
and the build would fail.

This list may not be complete. I mostly created it by some grep on
deprecated functions from libgimp 2.10.