leaflet: Start the child transition only when folded

This avoids starting the transition when unfolded, which doesn't make
sense as all children are visible.

Librem5/libhandy#35
1 job for leaflet-unfolded-transition in 1 minute and 54 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Build
passed #1970
librem5
build-debian

00:01:54