Add support for Tiberian Sun style right-click-and-drag scrolling

This patch introduces support for the right-click-and-drag scrolling that
is available in Tiberian Sun and Red Alert 2. It can be enabled by
selecting "Joystick" scrolling in the Input settings.

The speed of the scroll is proportional to the product of the distance of
the drag, and the Scroll Speed selected in the Input settings menu.

A side-effect of this is that events previously tied to right clicks on
the world are now based on the release of the click rather than the press.

The "Middle-Mouse Scrolling:" option is renamed
to "Mouse Scrolling Method:"

OpenRA.Game/Settings.cs

@@ -19,7 +19,7 @@ using OpenRA.Traits;
 
 namespace OpenRA
 {
-	public enum MouseScrollType { Disabled, Standard, Inverted }
+	public enum MouseScrollType { Disabled, Standard, Inverted, Joystick }
 
 	public class ServerSettings
 	{
@@ -167,6 +167,7 @@ namespace OpenRA
 		public float ViewportEdgeScrollStep = 10f;
 		public float UIScrollSpeed = 50f;
 		public int SelectionDeadzone = 24;
+		public int JoystickScrollDeadzone = 8;
 
 		public bool UseClassicMouseStyle = false;
 		public bool AlwaysShowStatusBars = false;