Experimental GUI tweaking: skip selection, if entities already selected

Hi,

I'm not sure this is a good idea, but this feature works:

1, select something;
2, Ctr-C
-, Omitted: select entities to copy
3, select reference point;
4, Ctr-V
5, select target point;

Similar cases for Rotate, Mirror, etc.

commit: https://github.com/LibreCAD/LibreCAD/commit/872e3e52ae0f8c21f5fbeca14fc38a2a8a89717c

I like it.