In convergence mode the separate navigation panel is redundant
with window title-bar controls. Remove it by setting its
thickness to zero and visibility to false, then embed Home and
Overview buttons at the left and right ends of the favourites
bar. Running application icons with context menus are shown
between the favourites and the Overview button.
Add HomeScreen::triggerOverview() to invoke the KWin Overview
shortcut over D-Bus so the homescreen containment can open it
without access to the task-panel Plasmoid.
