SetSelection functions to control output highlight

Like the opposites of GetSelectionStartColumn, GetSelectionEndColumn, GetSelectionStartLine, and GetSelectionEndLine. Instead of reporting what is highlighted, they would control what is highlighted.

Why would you want to do this? If I am sitting at my keyboard I don't expect text to suddenly be selected while I am gazing out the window at the unicorns.

It would be based on actual user interaction in a way that makes sense.

I've written a splitscreen scrollback plugin that shows the most recent text as a native-looking overlay over the bottom section of the output when the player scrolls up, and it allows text highlighting using the same code that I developed over the years for my chat capture miniwindow. If a player highlights some text in the main output and then scrolls up, I can currently propagate their highlight into the miniwindow so that they still see it active, but if they highlight something in the miniwindow I have no way to propagate that back to the main output (both for access to native Ctrl+C copying and also so that the highlight is still there when they scroll all the way to the bottom and the overlay goes away).

Here's a video showing what it currently looks like: https://gofile.io/?c=WQ2JJ1

Added to pre-release version 5.07.

https://github.com/nickgammon/mushclient/commit/0d02b24

Yay