Fix real world submenus (e.g. kate) not getting their first item selected on open
MenuBar.qml: remove unused function MenuPopup.qml: Select first item when one arrives if there's no currentItem instead of on completion unitymenumodel.cpp: trick rowCount() to simulate "real world late" menus tst_MenuPopup.qml: adapt to unitymenumodel.cpp changes (and make the Up test actually press Up and not Down) (LP: #1666859)