app: fix config properties / spin buttons mapping for Convolution Matrix filter

The filter's output is now correct, and rotating/flipping the matrix works as
expected.
16 jobs for master in 33 minutes and 45 seconds (queued for 1 second)
Status Job ID Name Coverage
  Prepare
passed #1117918
build-image

00:01:36

passed #1117920
build-image-win32

00:01:35

passed #1117919
build-image-win64

00:01:35

 
  Dependencies
passed #1117921
deps-debian

00:01:38

passed #1117923
deps-win32

00:08:14

passed #1117922
deps-win64

00:09:40

 
  Gimp
passed #1117924
gimp-autotools-debian

00:08:25

passed #1117927
gimp-clang-debian

00:04:02

passed #1117925
gimp-distcheck-debian

00:21:19

passed #1117926
gimp-meson-debian

00:03:24

passed #1117929
gimp-win32

00:08:15

passed #1117928
gimp-win64

00:14:27

 
  Analysis
passed #1117930
cppcheck

00:02:12

 
  Distribution
passed #1117931
sources

00:02:13

passed #1117933
win32-nightly

00:09:01

passed #1117932
win64-nightly

00:07:59