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.