[MM-46424] Fix 3-dot menu not losing focus after clicking outside (Windows)

[tboulis]

Summary

On Windows the 3-dot button(that opens the app menu) would not lose focus after closing the menu or clicking outside of it. The problem was that the BrowserView of the top bar(3-dot button, server dropdown, tabs) could not fire the focusout event on the 3-dot because it can't "see" outside of its bounds (when a user would click outside of it). To address that I used a state variable inside the MainApp component that changes the style of the button when it is hovered, or when the menu opens/closes.

I also tested that pressing the Alt button on windows focuses the 3-dot button

Ticket Link

https://mattermost.atlassian.net/browse/MM-46424

Device Information

This PR was tested on:

• Windows 10 Pro, version 21H2

Screenshots

https://recordit.co/huulXU3KQV

Release Note

NONE