applets/kicker: refresh RootModel on session backend state change

BUG: 427530

Hunting this one down since mid-March...

In "native" Plasma containments like the panel, desktop, and plasmawindowed, the `LogindSessionBackend` in `libkworkspace` enters the "Ready" state nearly instantly, before any signals can even be attached. In other containments like Latte Dock and plasmoidviewer, however, the initialization takes some time to complete (unsure why). RootModel was not watching or waiting to check what the session backend's state was, resulting in validity checks being done too soon and never checked again.


(cherry picked from commit 2cc819f3)