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.

src/msw/webview_ie.cpp

@@ -1360,7 +1360,7 @@ void wxWebViewIE::onActiveXEvent(wxActiveXEvent& evt)
             if(m_historyEnabled && !m_historyLoadingFromList &&
               (url == GetCurrentURL() ||
               (GetCurrentURL().substr(0, 4) == "file" &&
-               wxFileSystem::URLToFileName(GetCurrentURL()).GetFullPath() == url)))
+               wxFileName::URLToFileName(GetCurrentURL()).GetFullPath() == url)))
             {
                 //If we are not at the end of the list, then erase everything
                 //between us and the end before adding the new page