[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/SettingsForm.Designer.cs

@@ -1,4 +1,4 @@
-namespace Observatory.UI
+namespace Observatory.UI
 {
     partial class SettingsForm
     {
@@ -50,6 +50,7 @@
             Controls.Add(PluginSettingsCloseButton);
             FormBorderStyle = FormBorderStyle.FixedSingle;
             Name = "SettingsForm";
+            StartPosition = FormStartPosition.CenterScreen;
             Text = "SettingsForm";
             ResumeLayout(false);
         }