[dagit] Don't show shortcut hints on screen capture command
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.
Reviewers: bengotow, sashank, sandyryza
Reviewed By: sashank
Differential Revision: https://dagster.phacility.com/D5401