start migrating to tikz::Path with full undo/redo support

Started work on qtikzcore so far only. qtikzgui and unit tests are
missing. A lot of work ahead.
add Edge::setStyle()
more consistent naming
add support changing an Edge type on the fly
the curve mode moved to tikz::Edge
move enum tikz::ElementType to Edge::Type
compile & run again, pretty broken, though
push other changes
comment out unit test
introduce path inheritance
rename MetaNode to MetaPos

add Anchor MetaPos::anchor() const;
implement MetaPos::setAnchor
add class EdgePath
PathEdge: undo-aware support for dis/connecting edges
Path: move undo action + rename
bring UndoSetPathStyle back to life