leaflet: Merge the two transition types

Replace HdyLeafletModeTransitionType and HdyLeafletChildTransitionType
by HdyLeafletTransitionType. This makes the code simpler.

Fixes Librem5/libhandy#92
7 jobs for leaflet-transition-type in 13 minutes (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #58804
librem5
build-debian-clang

00:02:34

passed #58802
librem5
build-debian-gcc

00:02:55

passed #58803
librem5
build-debian-gcc-static

00:02:00

passed #58805
librem5
build-fedora-gcc

00:04:25

 
  Docs
passed #58806
librem5
build-gtkdoc

00:01:31

 
  Test
passed #58807
librem5
unit-test

00:03:00

46.4%
 
  Package
passed #58808
librem5
package-debian

00:02:43