Add option for flying back button

It has been discussed often in various places: reaching to the top left corner for "back" is considered cumbersome by many users - not by all though.

This MR adds an optional "flying back button" to places where it does make sense. By default this is off. The button can be reached very conveniently with the right hand thumb. The places where the button is used can be configured individually in settings page. So everyone is free to use it where they wish to use it. No one is forced to use it.

From my personal experience after using this for some months now, I don't want to live without it anymore. I wish more apps have it. 😏

Of course the button sits on top (visually) of some components. This is a downside, but in my opinion worth it. Again, no one needs to use it. In chats it does make the most sense, but for consistency I added it in the other places too. One does get used to it quickly...

Some screenshots:

1. Button in a chat
2. settings page
3. button in settings page
4. button in image preview