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.

include/wx/filename.h

@@ -258,6 +258,11 @@ public:
         // values
     bool SetPermissions(int permissions);
 
+    // Returns the native path for a file URL
+    static wxFileName URLToFileName(const wxString& url);
+
+    // Returns the file URL for a native path
+    static wxString FileNameToURL(const wxFileName& filename);
 
     // time functions
 #if wxUSE_DATETIME