allow [MODIFIER]+ESC shortcuts

only naked ESC without modifiers should cancel editing

http://discuss.binaryage.com/t/cant-change-activation-key-to-control-esc

repro case:
1. enter editing mode
2. press CTRL+ESC
3. editing gets canceled without shortcut change

expected behaviour: 
CTRL+ESC is set as new shortcut in step 3

tested with [TotalTerminal](http://totalterminal.binaryage.com/) codebase