workspace: Don't use grid layout

workspace: Don't use grid layout

Right now we arrange the window thumbnails in a grid if there are more than two
rows of them. This was originally intended to reduce the amount of noise in the
thumbnail arrangement. It also made sense when the thumbnails were of a similar
size.

Nowadays we reflect the size of windows in the size of the thumbnail. This
leads to huge amounts of space between some windows when they are grid aligned.
Removing the grid alignment would also lead to larger thumbnails.

https://bugzilla.gnome.org/show_bug.cgi?id=694210