leaflet: Consider having a single transition type

HdyLeaflet has two kind of transitions: child transitions represented by HdyLeafletChildTransition and mode transitions represented by HdyLeafletModeTransition. It makes sense for both transitions to have the same semantic, and we are moving towards implementing the same transitions to both types.

If the two transition types end up similar and should be similar in any case, we could merge them as we have the same transitions for both, we could merghe them into the single HdyLeafletTransition type.

I can't think of any case where it is desirable for the child and mode transitions to be different, se we probably also could merge the child-transition-type and mode-transition-type properties into a single transition-type property.

At this point, it's probably also worth asking if we want to keep the two child-transition-duration and mode-transition-duration properties, but I think the topic is orthogonal to the type issue and it's likely worth keeping them separate.

Any of these changes would break the API.