Make "Browse Addins" window keyboard friendly

[randy3k]

I mapped the "Browse Addins" to a key and hoped that I could execute an addin without moving the mouse (similar to the Command Palette of Sublime Text).

However, there are two issues here

1. The search box is not auto focused when the window is opened. It means that I need to use my mouse to manually focus the search box.
2. Typing in the search box and then hitting enter will dismiss the window, no matter if there is one result or more than one results.

[randy3k]

It's interesting that this issue falls into its own category.

[ronblum]

@randy3k This isn't to say that it's the only accessibility feature. For example, enabling DOM accessibility is available: Tools -> Global Options -> General -> Advanced -> Enable DOM Accessibility

This is just the first logged issue that's been identified as impacting accessibility.

[randy3k]

@kevinushey Thanks for the new commit. It would be also nice to allow tab to jump to the list of commands from the search field and allow the up and down keys to choose different commands.