Stop using gtk-missing-image

I finally got the build tests for glade a bit closer to running in Debian which uncovered a bug.

Please stop using the gtk-missing-image icon. GNOME considers it deprecated ever since they switched from gnome-icon-theme to adwaita-icon-theme and dropped a bunch of former "stock" icons.

Workaround

Install gnome-icon-theme

Build log excerpt

========================================
   glade 3.22.1: tests/test-suite.log
========================================

# TOTAL: 3
# PASS:  0
# SKIP:  0
# XFAIL: 0
# FAIL:  3
# XPASS: 0
# ERROR: 0

.. contents:: :depth: 2

FAIL: create-widgets
====================

GladeUI-Message: 12:46:46.008: 1 missing displayable value for GtkWidget::events
GladeUI-Message: 12:46:46.009: 1 missing displayable value for GtkWidget::AtkObject::accessible-role

(/<<PKGBUILDDIR>>/tests/.libs/create-widgets:31796): GladeUI-WARNING **: 12:46:46.032:
 Unable to render icon gtk-missing-image at size 16: Icon 'gtk-missing-image' not present in theme Adwaita
FAIL create-widgets (exit status: 133)

FAIL: add-child
===============

GladeUI-Message: 12:46:46.061: 1 missing displayable value for GtkWidget::events
GladeUI-Message: 12:46:46.062: 1 missing displayable value for GtkWidget::AtkObject::accessible-role

(/<<PKGBUILDDIR>>/tests/.libs/add-child:31797): GladeUI-WARNING **: 12:46:46.085: 
Unable to render icon gtk-missing-image at size 16: Icon 'gtk-missing-image' not present in theme Adwaita
FAIL add-child (exit status: 133)

FAIL: toplevel-order
====================

GladeUI-Message: 12:46:46.042: 1 missing displayable value for GtkWidget::events
GladeUI-Message: 12:46:46.043: 1 missing displayable value for GtkWidget::AtkObject::accessible-role

(/<<PKGBUILDDIR>>/tests/.libs/toplevel-order:31798): GladeUI-WARNING **: 12:46:46.070:
 Unable to render icon gtk-missing-image at size 16: Icon 'gtk-missing-image' not present in theme Adwaita
FAIL toplevel-order (exit status: 133)