[layers] Implemented layer insertion. No keys or menu yet, but it's already used when loading images, to make an image with right number of layers. Undoing/redoing such change works. Implemented single-layer deletion (untested)

git-svn-id: svn://pulkomandy.tk/GrafX2/branches/layers@1069 416bcca6-2ee7-4201-b75f-2eb2f807beb1

transform.c

@@ -368,7 +368,7 @@ void Button_Transform_menu(void)
     old_height=Main_image_height;
     
     // Allocate a new page
-    if (Backup_with_new_dimensions(1,new_width,new_height))
+    if (Backup_with_new_dimensions(1,Main_backups->Pages->Nb_layers,new_width,new_height))
     {
       // The new image is allocated, the new dimensions are already updated.