tikz::core::MetaPos now bound to a Document

This allows to derive tikz::ui::MetaPos from tikz::core::MetaPos.
All MetaPos objects created with Document::createMetaPos() will
later be of type tikz::ui::MetaPos() by overriding this function
in TikzDocument::createMetaPos().
add tikz:ui::MetaPos

and create ui::MetaPos objects in TikzDocument::createMetaPos()