[ObservatoryCore] UI tweaks

Polishing a few rough edges:
* Enable scrolling on the Plugin List view. It only fit 6 or 7. I have 13.
* I fiddled a bit with column auto-sizing (left it commented out, see comment).
* A whole bunch of tweaks to the settings view:
  * first row of controls was cut off at the top (under the window title bar)
  * fixing that revealed uneven spacing between columns
  * action buttons were teensy tiny and unreadable
  * settings view columns were too narrow (200 px).
  * added some spacing to help with readability
  * settings windows now start in center of current screen rather than some random place.

ObservatoryCore/UI/CoreForm.cs

@@ -212,6 +212,12 @@ namespace Observatory.UI
 
         private void FitColumns()
         {
+            // This sizes to fit the column width to the text. However, when the listview is resized (by
+            // expanding setting sections below the list), it causes flashing/jank. Visually, it looks cleaner
+            // (because the default column sizes are too small) and helps avoid horizontal scrollbar.
+            //foreach (ColumnHeader col in PluginList.Columns)
+            //    col.Width = -2;
+
             int totalWidth = 0;
             foreach (ColumnHeader col in PluginList.Columns)
                 totalWidth += col.Width;