Added API and menu item for auto-layout of nodes.

No implementation whatsoever yet.
WIP.
WIPWIP
WIPWIPWIPWIP
constraintsOnlyTidyup1
constraintsOnlyTidyup2
constraintsOnlyTidyup3
pinned flag - not doing anything yet
privatised constraints methods, removed m_constraints
Sibling sorting working vertically too