HdySwipeGroup

This is an RFC, adds a HdySwipeable interface (currently HdyPaginator implements it), and a HdySwipeGroup object for syncing multiple widgets together.

This will be necessary for swipeable HdyLeaflet later, to sync content and headerbar, but for now I have some doubts about HdySwipeGroup API.

1. Should the API use Swipeable or Widget naming, i.e. hdy_swipe_group_add_swipeable() or hdy_swipe_group_add_widget()? The second sounds a lot better to me, but it requires a HdySwipeable, not any random GtkWidget. (though swipeable must be widget) swipeable
2. Does it makes sense to have HdySwipeTracker use HdySwipeable internally? it's fine
3. Currently HdySwipeGroup is derivable, just like HdyHeaderGroup was. I seriously doubt it needs to be derivable. :) (both of them, really, but changing HdyHeaderGroup at this point would be an ABI break) final

There's a rudimentary example in the libhandy demo, will remove it later.

(yes, I know it's missing docs and tests atm)