Resolves #3012

Modify the shortcut keydown listener so that we stop showing shortcut hints when using commands like Cmd+Shift+4 to take screen captures.

The behavior will be:

• Any modifier pressed for more than 800ms will display the hints
• If any other key is pressed, including other modifiers, the timeout will be cleared and the hints will not be displayed

Verify that I can use Cmd+Shift+4 to take a screen capture, with no shortcut hints showing up.

Also verify that shorcuts still behave properly, and that holding down a modifier key will show the hints after 800ms.