(svn r1497) -Fix: [1101179] Crash if generating land while industry window is open. This also happened for towns and the land information window

-Added DeleteWindowByClass() function that deletes all windows of a given class

window.c

@@ -262,6 +262,18 @@ void DeleteWindowById(WindowClass cls, WindowNumber number)
 	DeleteWindow(FindWindowById(cls, number));
 }
 
+void DeleteWindowByClass(WindowClass cls)
+{
+	Window *w;
+	for (w = _windows; w != _last_window;) {
+		if (w->window_class == cls) {
+			DeleteWindow(w);
+			w = _windows;
+		} else
+			w++;
+	}
+}
+
 Window *BringWindowToFrontById(WindowClass cls, WindowNumber number)
 {
 	Window *w = FindWindowById(cls, number);