extensions, po-plug-ins: fix localization of extension metadata.

The metadata format being appdata, I simply use standard intltool
merging on the XML templates.
For Automake, I use a custom rule instead of INTLTOOL_XML_RULE because
the m4 macro hardcoded $(top_srcdir)/po as translation source directory
(yet we want to use po-plug-ins/ for these). Meson on the other side
already used a custom command.
16 jobs for master in 43 minutes and 7 seconds (queued for 1 second)
Status Job ID Name Coverage
  Prepare
passed #934259
build-image

00:02:23

passed #934261
build-image-win32

00:02:16

passed #934260
build-image-win64

00:02:17

 
  Dependencies
passed #934262
deps-debian

00:02:22

passed #934264
deps-win32

00:12:33

passed #934263
deps-win64

00:16:32

 
  Gimp
passed #934265
gimp-autotools-debian

00:09:41

passed #934268
gimp-clang-debian

00:07:51

passed #934266
gimp-distcheck-debian

00:27:04

passed #934267
gimp-meson-debian

00:16:06

passed #934270
gimp-win32

00:11:58

passed #934269
gimp-win64

00:13:44

 
  Analysis
passed #934271
cppcheck

00:04:22

 
  Distribution
passed #934272
sources

00:01:20

passed #934274
win32-nightly

00:11:35

passed #934273
win64-nightly

00:10:30