PropertySetModelImpl holds set of property sets and
properties for the new property sheet, manages their
expanded/collapsed state. Events on it get translated
to TableModelEvents by SheetTableModel.
The current implementation is a bit naive, as it was
just written to get the initial impl of the property
sheet working - it iterates all the property sets and
all the properties of them and puts them into a list
Probably for performance, the implementation should be
more transparent and not iterate/cache information.
Any rewrite should wait pending revisions of API for
properties wrt Looks - and there don't seem to be any
significant performance problems here anyway.