Untie BaseTree from VirtualTrees

[Fr0sT-Brutal]

There's possibility to reduce number of circular links by untying BaseTree from VirtualTrees. It would require:

1. adding empty method DoPaintText to BaseTree and call it unconditionally in TVirtualTreeHintWindow.CalcHintRect (or adding some flag to TVirtualTreeHintWindow)
2. calling RegisterVTClipboardFormat related to TCustomVirtualStringTree from VirtualTrees

Benefit of this change is not only reducing circular links but also extracting independent layer of BaseTree with its dependents and DrawTree independent from StringTree with Export, Accessibility and EditLink thus reducing binary size.

[joachimmarder]

adding empty method DoPaintText to BaseTree and call it unconditionally in TVirtualTreeHintWindow.CalcHintRect

Yes, that makes sense. We have to move TVSTTextType to unit VirtualTrees.Types, but that seems acceptable.

[joachimmarder]

calling RegisterVTClipboardFormat related to TCustomVirtualStringTree from VirtualTrees

Done.