To explain to the user what is happening, instead of abruptly changing
updating the workspace thumbnail list, slide thubmnails in and out
as they are added and removed.
To implement this, we track a state for each thumbnail and when things
change go through a process of first sliding removed thumbnails out,
then ollapsing the left-over spaces and rescaling the thumbnails, then
finally sliding newly added thumbnails in.
https://bugzilla.gnome.org/show_bug.cgi?id=641881
7598fd4581