applets/icon: prevent actions from being deleted too early

Those actions need to be alive until its parent class's dtor is run (Applet::~Applet) where signals connected to those actions are disconnected. Calling QObject::disconnect on deleted dangling pointers will cause a crash.

By reparenting those actions to IconApplet, the lifetime of those actions can be extended, which fixes the crash.

Closes https://crash-reports.kde.org/organizations/kde/issues/4914/