Fix Ctrl/Shift + insert copy/paste

Fix Ctrl/Shift + insert copy/paste

Many systems (including MS Windows and many linuxes) map `<Ctrl>` +
`<Insert>` to copy and `<Shift> + <Insert>` to paste. That serves as a
handy fallback when the more common `<Ctrl> + C` and `<Ctrl> + V`
keybindings have their default prevented to send signals to the
terminal.

Currently all keydown-events with the insert key send `\x1b[2~` to the
terminal. This commit won't send that key if either the `shiftKey` or
the `ctrlKey` are present. Instead it will enable `contentEditable` to
allow for pasting.