workspacesView: Work around spurious allocation changes

For some reason, people are still seeing those after commit d5ebd8c8.
While this is something we really should figure out, we can work around
the issue by keeping the view actors hidden until the update is complete.

GNOME/gnome-shell#1065
4 jobs for !625 with spurious-allocs-workaround in 5 minutes and 17 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Review
passed #352691
check_commit_log

00:00:21

 
  Build
passed #352692
build

00:03:56

 
  Test
passed #352693
test

00:01:00

passed #352694
test-pot

00:00:59