Create directory when saving the menu file

Commit 0bf70655 ported kmenuedit away from KStandardDirs.
But unlike KStandardDirs, QStandardPaths doesn't guarantee that the
target directory actually exists, therefore saving the menu file failed
if ~/.config/menus/ wasn't there already.

BUG: 413079
FIXED-IN: 5.17.2
Differential Revision: https://phabricator.kde.org/D24996