Introduce workspaces to replace tags

Introduce workspaces to replace tags

Tags are a nice idea, but they were clearly not designed for environments
with multiple monitors. For example, in an environment with three screens,
moving a window from one screen to another can require as many as 7 key-
strokes (assuming neither the source nor the destination tag are selected).

To make the grouping of windows more intuitive, this commit introduces
workspaces. In simple terms, a workspace is a screenful of windows (or
monitorful of clients, in X parlance). There are 12 workspaces in the
default configuration. Unlike tags, there is only one set of workspaces,
and all monitors can display the same workspaces. Meaning if I have a set
of windows that I want to display on a different monitor, I can point that
monitor to the workspace and it will display the windows without the need
to move around windows.
Better yet, because layouts are still set on a per-monitor basis, it is
possible to use different monitors to view the same windows in a different
layout. However, note that two monitors cannot display the same workspace
at the same time, since that would mean that a window has two different
sizes and positions at the same time. Pointing a monitor to a workspace
that is being displayed on another monitor will make the monitors swap
workspaces instead.