move some mode names into their own module
add EDIT_MODE_ANY
update mousePressEvent to pass to super if in edit mode
set flags on polygon item to make it movable, selectable and focusable
also give the text item a parent so it moves with everything else
make PolygonInfo subclass QGraphicsPolygonItem to make better use of
selecting, moving, etc. Objects aren't actually updated yet after
moving