appDisplay: Add :gesture-modes property

Merged Florian Müllner requested to merge fmuellner/gnome-shell:app-gestures into master

Both the main app grid and folders got a swipe tracker, which is active in both OVERVIEW and POPUP mode. The result is that two trackers fight over the same events, and everybody loses.

Address this by adding a :gesture-modes property that determines the allowed modes for the associated swipe tracker.

Fixes #4256 (closed)

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