Add built-in option to change title bar buttons

## Problem

For many users, default window buttons location can be confusing because of their previous experience on Windows or macOS.

## Proposal

Some beginner-friendly distros, like ZorinOS or Ubuntu Budgie (as other distros with Budgie being a default DE as well) provide an option to change window title bar buttons location to the left or right in settings:


And I think elementary OS should adopt this approach: use the current window buttons layout by default, but provide an easy-accessible option to change it to one used on Windows or macOS. This will be very helpful for people who have already used computer in the past, but starting to use elementary OS just now.

Sure, there are some third-party tools and config editors that allow to do the same (if not more), but the regular user likely won't even know how to use them.

## Prior Art (Optional)

The concept is based on a little older switchboard UI, as the last interface change uses more vertical space, which makes it look too stretched with a new option added. I tried to make the new option's UX as intuitive as possible: there are 3 options, with the label containing the name of the most common OS where this one is present, and a picture of how it will look like.


## UPD

Out of interest, I also made this kind-of-a survey on r/elementaryos (available at [https://redd.it/uyatio/](https://redd.it/uyatio/)). Despite it is still ongoing, the current results show that **more than 2/3** of participants use non-default window title buttons!

What I didn't expected though is the one more option (default WITH minimize icon) is popular as well, so perhaps there should be 4 options for most popular cases. I made a few newer concept arts for this, and also made the selection buttons a little more compact.


## UPD 2

The survey is now finished, and by its results only 33% of voters use default elementary layout, so implementing this option makes even more sense, as for now there's no any built-in tools to select one of the most popular custom layouts via GUI. The visualization of the results available is at https://ibb.co/hBJM47Z