Shortcut for Export

[jameskerr]

Adds the shortcut cmd+shift+e to pull up the export dialog.

[philrz]

@jameskerr: Looks great. Alas, because you've now made it so easy to quickly call up Export via the keyboard, I found myself instinctively hitting the arrow keys to move between the formats and Esc to dismiss without exporting and found a couple new bugs unrelated to this specific branch. If you'd rather I open new issues we could get to them later. But if they're easy and you're keen to fix them in this PR while you're under the hood, I would not fight that, as I suspect users named Jason might bump into them. 😄

First, I found that if I bring up Export and then immediately hit Esc, I can't seem to bring up Export again. I don't see any errors in DevTools.

Escape.mp4

Second, I found that if I bring up Export and then immediately click up/down arrows to switch formats, it jumps somewhere other than I'd expect relative to the default ZNG selection. Specifically, hitting up-arrow jumps me down to ZSON, and hitting down-arrow jumps me up to CSV.

Arrows.mp4

[jameskerr]

Closing this in favor of #2985 That PR will address these bugs.