Page Sets; Better automatic resizing when using columns

A Page Set consisting of multiple columns with no Page Objects positioned after the last column break will now resize any Page Objects with a Height set to "Size to fill empty space on the page" using the page height rather than the height of the tallest column.

Page Objects within a column that are set to auto-size their height will now apportion any remaining space between them; previously, only the last Page Object would recieve the left-over space.

Additional warnings are now given on the Pages page of the Page Sets form if a column contains the incorrect number of column breaks or a column break exists outside of a column since, if no ending column break exists (one should be added for each column defined in the Columns Start), resizing may be affected.