Add macOS + Windows global hotkeys

[karlhorky]

Closes #1593

[mmahmoudian]

Thanks for the contribution and it is very welcome. But your changes about Linux was so wrong that got me into rant mode, so I apologize but here it is:

On Linux because of proper and correct design of the operating system and desktop environment, user is not forced to use a keyboard shortcut and they can/should configure it themselves. As the matter of fact all keyboard shortcut complaints we have had so far in Flameshot were almost exclusively from Windows and Mac users due to their extremely limited and poorly-designed operating system.

Considering that we can never implement a code that force a shortcut to user in most Linux machines, I would consider re-phrasing the sentence.

[karlhorky]

Hm, I guess I would suggest not including personal views of superiority of different operating systems in documentation of software. I wouldn't know where to start to encode this viewpoint into docs.

But if that's the opinion of the whole maintainer team, then feel free to change it - edits by maintainers to this PR and flameshot-org/flameshot-org.github.io#31 are on.