Add 'Lock Screen' widget

[tresni]

Touchbar widget that locks the screen.

[billziss-gh]

@tresni thank you for the PR. This looks great.

I will merge it in, but I am wondering if it would work better as a button or at least if the icon was a bit brighter. What do you think?

[tresni]

@billziss-gh I can't figure out why the icon is dulled.. I thought maybe the template image was not using RGB(0,0,0) or something tweaked on alpha, but that didn't appear to be the case. It could be a button vs just the icon, I just wanted to keep it small and like the look of just the lock, but I'm not tied to it ;-)

[billziss-gh]

I can't figure out why the icon is dulled

I think the reason is that we are using the lock as a template image. If we made the icon white and did not use it as a template image it will likely no longer appear dulled.

I am happy to keep it as a non-button as I do not have a strong preference either. I suggested that it should be a button for consistency with the other buttons/controls.