app: add gimp_display_shell_show_image()

... which controls whether or not the image is rendered by the
shell.  We'll use this to hide the image while showing its
transform preview in the next commits.

(cherry picked from commit 539d666ae2)

app/display/gimpdisplayshell.h

@@ -91,6 +91,8 @@ struct _GimpDisplayShell
 
   gboolean           proximity;        /*  is a device in proximity           */
 
+  gboolean           show_image;       /*  whether to show the image          */
+
   Selection         *selection;        /*  Selection (marching ants)          */
 
   GList             *children;
@@ -297,6 +299,10 @@ gboolean          gimp_display_shell_mask_bounds   (GimpDisplayShell   *shell,
                                                     gint               *width,
                                                     gint               *height);
 
+void              gimp_display_shell_set_show_image
+                                                   (GimpDisplayShell   *shell,
+                                                    gboolean            show_image);
+
 void              gimp_display_shell_flush         (GimpDisplayShell   *shell,
                                                     gboolean            now);