Fix #10987: Double-close of dropdown stopped land-info tool working as default. (#11000)

Clicking and releasing on the query toolbar icon is meant to select the land-info tool.

This did not work as during closing a window, OnFocusLost() is called, which then closes the window again. These two calls toggled the land-info tool one and off in the same action.

Resolve by not calling Window::Close in OnFocusLost() if the window is already closing.

src/window_gui.h

@@ -515,9 +515,16 @@ public:
 	 */
 	virtual void SetStringParameters(int widget) const {}
 
+	/**
+	 * The window has gained focus.
+	 */
 	virtual void OnFocus();
 
-	virtual void OnFocusLost();
+	/**
+	 * The window has lost focus.
+	 * @param closing True iff the window has lost focus in the process of closing.
+	 */
+	virtual void OnFocusLost(bool closing);
 
 	/**
 	 * A key has been pressed.