Overhaul ActiveText.UMain unit

Add new action elements:
  ekDocument - wrapper for whole active text document
  ekBlock - generic block element

Changed rules of which elements can be permitted as as children of other
elements. The new action elements help to simplify the rules. In turn,
the simplification of rules makes writing parsers and renderers easier.

Revise IActiveText & TActiveText

Add new IActiveText.IsValidActiveTextDocument method with implementation
in TActiveText

Rewrite TActiveText.Append to produce a valid active text document
topped and tailed with ekDocumnent elements

Author: delphidabbler