#16 copy-buffer is cleared when z-ordering

[Mj Mendoza IV]

to recreate:

1. copy shapes from frame 1
2. Add Frame
3. paste copied frame
4. re-arrange shapes' z-ordering (with Object->Lower To Bottom)
5. try pasting again -- nothing happens because copy buffer is-cleared

[Mj Mendoza IV]

Fixed in https://sourceforge.net/p/kage/code/742/