plugins/overview: Allow switching between modes using shortcuts while already active
Overview and Grid modes have shortcuts assigned to them. While they provide a way to toggle the overview effect between on and off state, in other words overview <-> off or grid <-> off, it's not possible to move between the modes by pressing those shortcuts, e.g. off -> overview -> grid -> overview -> grid -> ... -> off.
The culprit seems to be that EffectTogglableState has two "inactive" states - Inactive and Stopped. It's counter-intuitive and needs a further cleanup.
To make switching between overview modes work, this change makes EffectTogglableState::toggle() toggle the state based on the Active state. There's only one active state.