Menu: Fix QPA related crash on exit

In QPA world, menus are always referenced by a menu item. (This
is the menu item either on the menu bar or in the parent menu.)
So, the same way that we set the menu item's menu in the menu's
constructor, we must clear it. Specially since qtbase commit
8605f44097a986e10c2ddaf4b4eae0b2331d98d1 which checks for the
menu reference in the menu item's destructor.

Change-Id: Id196fc7c2082dbff1d9006cbd82b7237532c4e3c
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
FileDialog: update icon whenever the filename changes

view.model.get(styleData.row, "fileIsDir") is unreliable for some
reason, but each filename usually changes when we go into a different
directory.  Maybe it could still fail if you navigate from a directory
containing a directory, to another directory which has a file with
exactly the same name in the same position.  Since most files have
extensions, this is unlikely...

Task-number: QTBUG-38683
Change-Id: If080c4f1a7a01b20f6c468fcb06653e3c2314a1a
Reviewed-by: Liang Qi <liang.qi@digia.com>
Reviewed-by: J-P Nurmi <jpnurmi@digia.com>
Merge remote-tracking branch 'origin/release' into stable

Change-Id: Ic5c4ea467890e1033e33161a6cf7f83cf5adb01c