leaflet: Allow editing the children list when looping through it

This avoids potential crashes when destroying a leaflet and this avoids
leaks as not all children where looped through as the children list was
edited while being looped through when destroying the leaflet.

This fixes Librem5/libhandy#42.
4 jobs for leaflet-fix in 3 minutes and 35 seconds (queued for 3 seconds)