leaflet: Add the under child transition animation

Let a and b be consecutive sibblings of the leaflet, a coming before b. → means the leaflet reveals a following sibbling. ← means the leaflet reveals a previous sibbling.

Currently we have the following child transition animations:

• none
• crossfade: similar to crossfade from GtkStack
• slide:
  • → a is pushed to the start by b
  • ← b is pushed to the end by a
  • similar to slide-left-right and slide-up-down from GtkStack
• over:
  • → a is covered by b coming from the end
  • ← b reveals a by moving towards the end
  • similar to over-left-right and over-up-down from GtkStack

I suggest to add the following animation:

• under:
  • → a reveals b by moving towards the start
  • ← b is covered by a coming from the start
  • similar to under-left+over-right and under-up+over-down from GtkStack