build: Switch from intltool to gettext

See https://gitlab.xfce.org/xfce/xfce4-dev-tools/-/issues/41.

.gitignore

@@ -1,12 +1,15 @@
 po/*.gmo
-po/.intltool-merge-cache*
 *Makefile
 *Makefile.in
 po/Makefile.in.in
+ABOUT-NLS
+ChangeLog
 POTFILES
 stamp*
 */.deps
 .libs
+*~
+*.bz2
 *.orig
 *.o
 *.lo
@@ -16,7 +19,6 @@ stamp*
 configure
 config.*
 configure.ac
-intltool*
 mkinstalldirs
 missing
 compile
@@ -26,6 +28,16 @@ libtool
 depcomp
 aclocal.m4
 m4/
+po/Makevars.template
+po/Rules-quot
+po/boldquot.sed
+po/en@boldquot.header
+po/en@quot.header
+po/insert-header.sin
+po/quot.sed
+po/remove-potcdate.sed
+po/remove-potcdate.sin
+po/xfce4-taskmanager.pot
 autom4te.cache
 *.desktop
 INSTALL