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 spacialization issues.
This also deprecates the two previous properties and their respective types, and drops the newly added over and under values of HdyLeafletModeTransitionType as there is no point adding them and deprecating them in the same version.
Fixes #92 (closed).