overviewControls: Support double-super when animations are off

When super is pressed again during the overview transition, we shift up to the app grid. That means that the feature currently doesn't work when animations are disabled (like in a VM), because there is no transition in that case.

Address this by adding a time-based fallback in that case, i.e. shift up when a second super-press occurs within 250ms after the first one.

Fixes #4121 (closed)

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