Added BuildableItemsChanged event to tech tree.

This lets the Sidebar only repopulate its item list when the items that can be built changes.

OpenRa.Game/SidebarItem.cs

@@ -9,6 +9,8 @@ namespace OpenRa.Game
 		public readonly float2 location;
 		readonly Sprite sprite;
 
+		public bool isBuilding = false;
+
 		public SidebarItem(Sprite s, Item item, int y)
 		{
 			this.techTreeItem = item;