swipeable: Add get_swipe_tracker()

Stop using g_object_get_data() for this, and make the API clearer by
actually requiring HdySwipeable implementation to have a swipe tracker.

Signed-off-by: Alexander Mikhaylenko <alexm@gnome.org>
12 jobs for swipe-tracker-public in 27 minutes and 33 seconds (queued for 1 second)
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  Build
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flatpak

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  Package
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  Test Package
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passed lintian-debian-buster-package #789859

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