Add #6887: Option to show zone inside local authority boundary of towns

Can be found at town information > local authority window
Layout for button is same as Graph Keys
Turn on/off for every town individually

src/core/kdtree.hpp

@@ -372,6 +372,17 @@ public:
 		CheckInvariant();
 	}
 
+	/**
+	 * Clear the tree.
+	 */
+	void Clear()
+	{
+		this->nodes.clear();
+		this->free_list.clear();
+		this->unbalanced = 0;
+		return;
+	}
+
 	/**
 	 * Reconstruct the tree with the same elements, letting it be fully balanced.
 	 */