[naokatvv] Splitting and merging rectangles

Consider operations of splitting and merging rectangles to form new rectangles.  Motivation is for a tiling window manager.

Nicer subset is hierarchical: a rectangle can be divided into multiple like a tic tac toe grid.  Merges only permitted if nodes share the same immediate parent.

Not hierarchical: arbitrarily split lines, almost arbitrary merges: a tricky task to decide what may be merged and form a rectangle.

Interesting operations: extend line (perhaps rejoining with an original line that got split).  Shift line (or only a portion of a line, perhaps non connected).  Shift vertex, moving two lines at once.  Proportional shift: shift a line keeping proportions within a region the same.