Onion service authorization prompt's key field does not get focus when clicked

If you click a different window while the onion service authorization prompt is displayed, there's no way to get its text field back in focus to enter the key without explicitly focusing the parent browser window first.

This can get extremely annoying because the popup stays on top of all other windows, which makes retrieving its parent quite frustrating.

To reproduce,

1. Open http://pierovlcy7baatz7xcaccbf2kanct3hvkhcgedz4cbrmcg2ybmjalxad.onion/ and wait for the authorization prompt
2. Move the focus to a different window
3. Click back onto the prompt (which should be visible because is rendered on top of all the other windows)
4. Try to type into the "Enter your privae key for this onion service" text field

/cc @pierov , @donuts