Major improvements to unit handling.
Major improvements to unit handling.

* The TUnit reference is stored in to convenient locations.
  One in the TabSheet.TagPointer and the other in the
  TreeNode.Data property.
* Loading a project correctly opens the previous opened units.
* Correctly handles File > Open function.
* Correctly handles Add unit to Project, and sets convenient
  references

Project management is now working well for the first time! :-)

wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI,wkitty42/fpGUI