po-windows-installer: fix double escaped ampersand in XML for the…

… Windows installer localization.

There are kind of 2 separate bugs here:

- Direct i18n.gettext() to the proper data directory where to find the
  ITS file. Otherwise `meson compile gimp30-windows-installer-pot` and
  `meson compile gimp30-windows-installer-update-po` complained about
  not knowing XML and falling back to C, which is obviously a problem:

> /usr/bin/xgettext: warning: file 'build/windows/installer/lang/setup.isl.xml.in' extension 'xml' is unknown; will try C

- Set gt:escapeRule to "no" in the ITS file, otherwise the XML entity is
  kept as-is in the po file (i.e. "&" stays "&" inside the po
  files), but it's considered as raw text when merged back to XML, i.e.
  that the '&' is properly converted to a XML entity, so we end up with
  a double escape "&".
  Now the po file will have a '&' which will still be converted to XML
  entity at merge time. This is actually most likely better than asking
  translators to handle XML entities themselves (with the possibility to
  make typos and break the XML entity).

See https://savannah.gnu.org/bugs/?58643

build/windows/installer/lang/its/gimp-ms-installer-config.its

@@ -3,4 +3,5 @@
            xmlns:gt="https://www.gnu.org/s/gettext/ns/its/extensions/1.0">
   <its:translateRule selector="/gimp-ms-installer-config" translate="no"/>
   <its:translateRule selector="//value" translate="yes"/>
+  <gt:escapeRule selector="//value" escape="no"/>
 </its:rules>

po-windows-installer/meson.build

@@ -1,2 +1,3 @@
 po_windows_installer_dir = meson.current_source_dir()
-i18n.gettext(gettext_package + '-windows-installer', preset: 'glib', install: false)
+i18n.gettext(gettext_package + '-windows-installer', preset: 'glib', install: false,
+             data_dirs: meson.source_root() / 'build' / 'windows' / 'installer' / 'lang')