SPARK-1888: Uninstall a plugin when the GUI instructs so.

This is a fix to what I assume to be a bug. The method, used by the UI part where plugins are managed,
is documented as 'remove and uninstall' a plugin. It only removed, but did not uninstall. I've fixed that
with this commit.

core/src/main/java/org/jivesoftware/spark/PluginManager.java

@@ -1027,6 +1027,7 @@ public class PluginManager implements MainWindowListener
         {
             if ( plugin.getName().equals( publicPlugin.getName() ) )
             {
+                uninstall( plugin.getPluginDir() );
                 publicPlugins.remove( plugin );
             }
         }