keyboard-select: deactive for Ctr-C

Hi,

I attempted to handle the Ctr-C key_press events, for two reasons: 
1. In Vim I used Ctr-C to deactivate modes, and I thought keyboard-select could be more Vim like in this regard.
2. If you accidentally activated keyboard-select or accidentally left it activated (this happens to me quite often, that I already switch tab with tabbed, and can't see that keyboard-select is active on another tab)
So if you are unaware of keyboard-select being active and you want to send a signal to a process running in the foreground, you keep pressing Ctr-C but nothing happens. Now first keyboard-select deactivates, and then the second sends the SIGINT.

I hope this modification doesn't mess up or conflict with anything.