[applets/showdesktop] Refactor peek/minimize controllers, and bind actions' text

Factoring out an "interface" (if one can call it like that in QML) for
the two operation modes removes all the decision making from the main
file, and delegates things like description text and toggle() method to
implementations. It turns ?trinary: mess into a set of simple and
straightforward bindings.

Test plan: applet works correctly in both configurations, on X11 and
Wayland; it's menu actions have adaptive text labels, and triggering
the one that matches applet's mode (primary controller) also updates
the highlight (active applet) indicator.