General Settings: "Brave is not your default browser" setting

[bradleyrichter]

Functionality:
Button:
For each specific OS, the button may require a different action/script.

Mac OS - Utilize available API to toggle OS level setting, and show the standard dialog.
Dialog sample:

Windows: Either toggle the default browser setting and show the Windows standard confirmation dialog, or open the windows CP if the API is not available. (Win 7 vs 8 vs 10 for example)

Linux: Same as windows.

Switch:
"Always check on startup" is on by default, but may be switched off by the user.

Item label:
"Brave is not your default browser"

Button label:
"Set as default..."

Switch label:
"Always check on startup"

Settings window:

Startup Sheet Dialog: (slides down from below tabs toolbar)

And setting toggle state: (when Brave is your default browser)

The button hides and the "startup check" switch moves up.

[darkdh]

Front end work is finished in https://github.com/brave/browser-laptop/tree/default-browser
And it works for Mac now. Working on electron API for windows and linux.