Add: Filter box to the save and load windows (#6974)

Save and heightmap names can be filtered.

src/fios.h

@@ -18,6 +18,13 @@
 #include "network/core/tcp_content.h"
 
 
+/** Special values for save-load window for the data parameter of #InvalidateWindowData. */
+enum SaveLoadInvalidateWindowData {
+	SLIWD_RESCAN_FILES,          ///< Rescan all files (when changed directory, ...)
+	SLIWD_SELECTION_CHANGES,     ///< File selection has changed (user click, ...)
+	SLIWD_FILTER_CHANGES,        ///< The filename filter has changed (via the editbox)
+};
+
 typedef SmallMap<uint, CompanyProperties *> CompanyPropertiesMap;
 
 /**