css: Add a -gtk-icon-theme CSS property

The property is useless to set (it only allows 'initial', 'inherit' and
'unset' as values), but it is used to track changes to the icon theme.
And as such, it can ensure that widgets can track when they need to
reload icons.

https://bugzilla.gnome.org/show_bug.cgi?id=743341

gtk/gtkcssstylepropertyimpl.c

@@ -45,6 +45,7 @@
 #include "gtkcsscornervalueprivate.h"
 #include "gtkcsseasevalueprivate.h"
 #include "gtkcssenginevalueprivate.h"
+#include "gtkcssiconthemevalueprivate.h"
 #include "gtkcssimageprivate.h"
 #include "gtkcssimagebuiltinprivate.h"
 #include "gtkcssimagegradientprivate.h"
@@ -945,6 +946,15 @@ background_position_parse (GtkCssStyleProperty *property,
   return _gtk_css_array_value_parse (parser, _gtk_css_position_value_parse);
 }
 
+static GtkCssValue *
+icon_theme_value_parse (GtkCssStyleProperty *property,
+		        GtkCssParser        *parser)
+{
+  _gtk_css_parser_error (parser, "Only 'inherit', 'initial' or 'unset' are allowed");
+
+  return NULL;
+}
+
 /*** REGISTRATION ***/
 
 void
@@ -975,6 +985,15 @@ _gtk_css_style_property_init_properties (void)
 
   /* properties that aren't referenced when computing values
    * start here */
+  gtk_css_style_property_register        ("-gtk-icon-theme",
+                                          GTK_CSS_PROPERTY_ICON_THEME,
+                                          G_TYPE_NONE,
+                                          GTK_STYLE_PROPERTY_INHERIT,
+                                          GTK_CSS_AFFECTS_ICON,
+                                          icon_theme_value_parse,
+                                          NULL,
+                                          NULL,
+                                          _gtk_css_icon_theme_value_new());
   gtk_css_style_property_register        ("background-color",
                                           GTK_CSS_PROPERTY_BACKGROUND_COLOR,
                                           GDK_TYPE_RGBA,