GH-162: Initial implementation of overlay-based 'focus fix'.

[duhrer]

I had been intermittently seeing cases where focus styles were not displayed, this pull attempts to provide a workaround for the underlying issues.

See #162 and #161 for more details on the research and the browser behaviour that required the workaround.

[duhrer]

As mentioned in the issue, the easiest way to see the underlying problem is to launch a search with the action launcher. Bumper navigating through the results does not display anything for multiple "bumps", and then finally only displays the destination URL for focused links.

With the new setting on, it's much clearer that menu elements, etc. are focused during this "invisible" period. There are two ways to enable the new setting:

1. Open the settings panel, it's the last entry under "General Preferences".
2. This is the first and only setting with a dedicated action to "toggle" it, so you can also open the action launcher and click "Toggle optional focus indicator".

[duhrer]

In working on this, I also figured out that although the input mapper listens for local storage changes, the settings panel didn't. I fixed this, so if you change the preference through the action launcher or through the settings panel, it gets updated in any other open windows we control.