Save and restore grid column widths

Now preserving column widths for the three DataGrids and the main
ListView.  In theory the various grids would conveniently auto-size
to the content, but in practice that doesn't work well with
virtualization.

There is, of course, no simple "the width has changed" event
provided by the control.  On the plus side, you can attach a
property-change event handler to pretty much anything, so once you
know the trick it's possible to make everything work.  Yay WPF.

CommonWPF/WindowPlacement.cs

@@ -1,15 +1,16 @@
 /*
- * This comes from "David Rickard's Tech Blog", posted by RandomEngy on March 8 2010:
+ * This comes from a blog entry, posted by RandomEngy on March 8 2010:
  * https://blogs.msdn.microsoft.com/davidrickard/2010/03/08/saving-window-size-and-location-in-wpf-and-winforms/
+ * (see https://stackoverflow.com/a/2406604/294248 for discussion)
  * 
  * Saving and restoring a window's size and position can be tricky when there are multiple
  * displays involved.  This uses the Win32 system functions to do the job properly and
  * consistently.  (In theory.)
  * 
  * The code works for WinForms (save on FormClosing, restore on Load, using the native handle
- * from the Handle property) and WPF (use the Window extension methods in Closing and
- * SourceInitialized).  Besides convenience, it has the added benefit of being able to
- * capture the non-maximized values for a maximized window.
+ * from the Handle property) and WPF (use the Window extension methods provided below, in
+ * Closing and SourceInitialized).  Besides convenience, it has the added benefit of being able
+ * to capture the non-maximized values for a maximized window.
  */
 
 using System;
@@ -116,9 +117,11 @@ namespace CommonWPF {
         // Extension methods for WPF.
         //
 
+        // Call from Closing event.  Returns XML string with placement info.
         public static string GetPlacement(this Window window) {
             return GetPlacement(new WindowInteropHelper(window).Handle);
         }
+        // Call from SourceInitialized event, passing in string from GetPlacement().
         public static void SetPlacement(this Window window, string placementXml) {
             SetPlacement(new WindowInteropHelper(window).Handle, placementXml);
         }