icons: install PNG version of gimp-(error|info|question|warning) icons.

These icons are requested by the System theme, which outputs warnings at
loading, so they are needed even when using vector icons otherwise.
Not sure why these are hardcoded. Maybe we should rather edit these out
from the gtkrc instead. But this will do for the time being.

icons/icon-list.mk

@@ -1020,17 +1020,19 @@ icons48_images = \
 
 icons64_images = \
 	64/dialog-information.png				\
-	64/gimp-error.png					\
 	64/gimp-frame.png					\
-	64/gimp-info.png					\
-	64/gimp-question.png					\
 	64/gimp-texture.png					\
 	64/gimp-user-manual.png					\
-	64/gimp-warning.png					\
 	64/gimp-wilber-eek.png					\
 	64/gimp-wilber.png					\
 	64/gimp-wilber-outline.png
 
+icons64_system_images = \
+	64/gimp-error.png					\
+	64/gimp-info.png					\
+	64/gimp-question.png				\
+	64/gimp-warning.png
+
 icons96_images = \
 	96/gimp-wilber.png					\
 	96/gimp-wilber-outline.png
@@ -1058,7 +1060,8 @@ icons24_DATA = \
 	$(vector24_images)
 
 icons64_DATA = \
-	64/gimp-frame.png		\
+	$(icons64_system_images)	\
+	64/gimp-frame.png			\
 	64/gimp-texture.png
 
 scalable_DATA = \
@@ -1104,7 +1107,8 @@ icons48_DATA = \
 	$(icons48_images)
 
 icons64_DATA = \
-	$(icons64_images)
+	$(icons64_images) \
+	$(icons64_system_images)
 
 icons96_DATA = \
 	$(icons96_images)