[Core] Improve(?) Plugin List column sizing (#133)

After populating the list of plugins, auto-size the columns once. In the Resize event handler, do what you did before but account for the scrollbar width too. Prevents horizontal scrollbar appearing except for really fast resize movements.

I didn't try it yet with fewer than a scrollable # of plugins yet.

This cleans up my commented out code from #128.

ObservatoryCore/UI/CoreForm.Plugins.cs

@@ -32,6 +32,9 @@ namespace Observatory.UI
                     PluginList.Items.Add(item);
                 }
             }
+
+            PluginList.AutoResizeColumns(ColumnHeaderAutoResizeStyle.ColumnContent);
+            PluginList.AutoResizeColumns(ColumnHeaderAutoResizeStyle.HeaderSize);
         }
 
         private static string PluginStatusString(PluginManager.PluginStatus status)