D5806: [WIP] when building menu hierarchies, parent menus to their containers

[Marco Martin]

mart added reviewers: Plasma on Wayland, Frameworks.

REPOSITORY
  R263 KXmlGui

REVISION DETAIL
  https://phabricator.kde.org/D5806

To: mart, #plasma, #plasma_on_wayland, #frameworks
Cc: plasma-devel, #frameworks, ZrenBot, spstarr, progwolff, lesliezhai, 
ali-mohamed, jensreuterberg, abetts, sebas, apol, lukas