app: use "Save Image As" for the "Save As..." dialog title

Save uses "Save Image", so I opted for "Save Image As" for consistency.

This introduces an inconsistency with the Export and Export As... dialogs where "Export Image" remains in use
for both, but addressing that will require an API change for file_export_dialog_show()

Fixes issue #4021.

app/actions/file-commands.c

@@ -293,7 +293,7 @@ file_save_cmd_callback (GimpAction *action,
 
     case GIMP_SAVE_MODE_SAVE_AS:
       file_save_dialog_show (gimp, image, widget,
-                             _("Save Image"), FALSE,
+                             _("Save Image As"), FALSE,
                              save_mode == GIMP_SAVE_MODE_SAVE_AND_CLOSE, display);
       break;