Extract VST into VirtualTrees.StringTree.pas, leave VirtualTrees.pas as aggregate unit

[Fr0sT-Brutal]

Just an idea that won't change things much but will continue logical separation of various tree implementations.
As a downside - this probably will require a new entry in all packages

[joachimmarder]

I like the idea, although I think we already have enough breaking changes in the upcoming V8.

[Fr0sT-Brutal]

Well, if TVST will be redeclared in old unit, this won't be a breaking change AFAICS. Now VirtualTrees.pas contains only a few original public declarations as most of declarations are aliases. So just some more aliases. I'll make PR with this change for you to estimate required changes.

[joachimmarder]

Maybe I should have left out the word "breaking". I have no intention to make this change myself for V8.0, but feel free to send a pull request.