BC412: add to backend_tcl; add skeleton test

GUI: allow for Qt not parsing #if's in slots (copy_to_clipboard_png())

backend_tcl/configure

@@ -5567,6 +5567,7 @@ done
 	../backend/2of5.c
 	../backend/auspost.c
 	../backend/aztec.c
+	../backend/bc412.c
 	../backend/bmp.c
 	../backend/codablock.c
 	../backend/code128.c