leaflet: Unify the transition types

Add the HdyLeafletTransitionType enumeration and the transition-type
property to define both the mode and child transitions, as having them
different makes no sense and could lead to spatialization issues.

This new type doesn't offer a crossfade transition on purpose as it was
deemed inappropriate for the leaflet, for which the position of the
children is inherently important.

This also deprecates the two previous properties and their respective
types.

Fixes #92.