Fixup of test-gimpidtable.c of things sleepy eyes and brains missed.

It can just as well return NULL.

Add a "check-for-deprecated-procedures-in-script-fu" make target that
looks for usage of deprecated procedures in *.scm files. We currently
use quite a bit of deprecated procedures. When we don't do that
anymore, we should connect this to make check somehow so our nightly
build fails if someone deprecated a procedures without also porting
all clients.

Run it like this from the source root:

  make -C plug-ins/script-fu check-for-deprecated-procedures-in-script-fu

Allow to dump the names of all internal deprecated PDB
procedures. This enables us to do static analysis on *.scm files and
look for usage of deprecated procedures.

by adding the source outline's canvas item to the source display
directly. Poking in GimpDrawTool internals is gone and generally not
possible any longer since GimpCanvasItem based drawing.

Not written in Vala... ;)

use gdk_x11_window_foreign_new_for_display() instead

this is a followup to commit 844df2b4

instead of always passing FALSE when changing to a different
layer. For some reason we were displaying the right state anyway,
which can only mean that we do things redundantly here and call this
function twice.