Fix clipboard issue

[bartbroere]

Just now I was using the Quadratic spreadsheet but I didn't get copying and pasting to work while using the Dvorak layout. It didn't work for Ctrl + QWERTY's C location and for Ctrl + Dvorak's C location.

The interchanged use of event.code and event.key seemed intentional, but because Ctrl + QWERTY's C location on my pc translates to Ctrl + j. In Chrome this opens my Downloads view.

It might be less confusing if we don't have the shortcut as Ctrl + QWERTY's C location, but simply as Ctrl + c.

If we want to change this for the following key shortcuts, I could also do it for all the other shortcuts that may have unintended side effects.