#89 Cut-Paste: ignore empty selection

[Heiko Selber]

Wish: ignore Ctrl-c or CTRL-x if nothing is selected.

It happens quite often when I cut and paste with Ctrl-
x / Ctrl-v, that I accidentally hit Ctrl-C instead of Ctrl-v.

In IDLE, this empties the cut buffer.

Other editors ignore Ctrl-x in this case, leaving the
content of the cut buffer intact.