Move URL<->filename conversion functions to wxFileName

This ensures that they are always available and can be used in
wxLaunchDefaultBrowser() in all build variants, whereas before this
function didn't handle file:// URLs correctly when the library was built
with wxUSE_FILESYSTEM==0.

Closes https://github.com/wxWidgets/wxWidgets/pull/1469

Closes #10414.

interface/wx/filename.h

@@ -1246,6 +1246,25 @@ public:
     */
     bool SetPermissions(int permissions);
 
+    /**
+        Converts URL into a well-formed filename.
+        The URL must use the @c file protocol.
+        If the URL does not use @c file protocol
+        wxFileName object may not be good or may not exist
+
+        @since 3.1.3
+    */
+    static wxFileName URLToFileName(const wxString& url);
+
+    /**
+        Converts wxFileName into an URL.
+
+        @see URLToFileName(), wxFileName
+
+        @since 3.1.3
+    */
+    static wxString FileNameToURL(const wxFileName& filename);
+
     /**
         Sets the file creation and last access/modification times (any of the pointers
         may be @NULL).