Added new "View" menu, containing a "Column Width Mode" submenu which allows the user to choose between different column width sizing options. The following are what's now available:

"None" - default - allows the user to resize column widths manually
"Fill" - fill all columns to width of DGV
"Autofit Using All Cells" - all columns will be automatically resized to fit the content width of all cells in that column.
"Autofit Using Visible Cells" - all columns will be automatically resized to fit the content width of all visible cells in that column.

Resolves #45