Side-by-side Workspaces
(Split from !1547 (closed))
Implement side-by-side workspaces.
I initially had this handful of patches as part of the new layout manager, but realized they're actually unrelated.
(Click to switch workspaces is not included here because I want to reimplement it entirely. The cover actor approach is not fit for what we need there.)
Edited by Georges Basile Stavracas Neto