Create free form box layout definition

[Martin Howarth]

Allows definition based layout of boxes (in arbitrary ways rather than just rows and columns). Also add an edit mode where users can drag boxes and it prints out the new layout definition?

[Martin Howarth]

See also #60

Something like this?

Alignment = Tuple[HorizontalAlignment, VerticalAlignment, inside: bool, spacing: bool]
# inside is a bool - if True then box is placed inside parent according to alignment, otherwise placed outside of parent bounds

Box.add_child_with_alignment(child, alignment)

and

Box.add_sibling_with_alignment(sibling, alignment)

[Martin Howarth]

Fixed by #84