leaflet: Add methods to move forward/backward

That would allow to express "move back" or "move forward" rather than "move to that specific child", simplifying the implementation of navigation in the apps.

Most of the logic for that has been introduced when making HdyLeaflet a HdySwipeable.

Maybe we could even just have to bind the "move back" method to the "clicked" method of a back button in a UI template.

Edited Jan 18, 2020 by Adrien Plazas

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