#1 PaneNodes should remember if they are Expanded/Collapsed

[William Russell Morris]

PaneNodes should treat their Expanded property as a
locally stored attribute - not something derived from the
TreeNode they are associated with. Once this is done,
the following functionality should be present:

- PaneNodes/PropertyPanes remember their Expanded
value after being removed from a PropertyTree and then
added back into it.

- PropertyPanes should persist their Expanded state to
source code during design-time.

- Expanded should be gettable/settable regardless of
whether or not the PaneNode is currently in an actual
tree.