support for multilevel kcms

Summary:
Make the title a toolbutton, and when there are multiple levels, make
a breadcrumb out of buttons.
clicking on buttons will navigate the levels(keyboard shortcuts work too)

a similar modification will be needed to be done to KCMShell
depends from D14580

Test Plan:
Added a level to a kcm, works as intended
This latest version works correctly on both the sidebar and icongrid systemsettings mode

Reviewers: #plasma, #vdg, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: davidedmundson, abetts, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D14581