[Menus] Menus are not closed when opening links with middle mouse button

It is the same behaviour as in Konqueror
Closes #1165
[LocationBar] Don't incorrectly show about to be loaded url when it was not accepted

It can happen for example when trying to change address on a page that
handles window.onbeforeunload event
[Json] Removed dependency on QJson

QtScript is now used instead of QJson. QtScript is available on both
Qt 4 & Qt 5 and also as side benefit, is TWICE as fast as QJson!