emit a signal when webpage launching external url

Sometimes when webpage launching external url,  user maybe want to get a prompt dialog just like chrome, or get a choice to decide whether launch external application or not.

This means in src/core/web_contents_delegate_qt.cpp

void WebContentsDelegateQt::launchExternalURL(const QUrl &url, ui::PageTransition page_transition, bool is_main_frame, bool has_user_gesture)
{ 
........ 
if (navigationRequestAccepted)     
    QDesktopServices::openUrl(url); 
....... 
} 

it is better not call "QDesktopServices::openUrl" directly,  I suggest it should just emit a signal, let developer who use qtwebengine to handle it.