Fix menu item width when the delegate is overriden
Summary:
Have MenuItem get the width of the parent instead of needing to specify it
every time.
This fixes an issue where MenuItem is used from Kirigami.ActionsMenuItem where
it would sometimes not take the whole width. See bug.
BUG: 401792
Reviewers: Kirigami, mart
Reviewed By: Kirigami, mart
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D17484