workspacesView: Simplify the workspacesOnlyOnPrimary implementation

workspacesView: Simplify the workspacesOnlyOnPrimary implementation

Before, workspacesOnlyOnPrimary was implemented in quite a crazy manner:

 * If workspacesOnlyOnPrimary was false, we'd create one WorkspacesView per
   monitor, with the primary one being a bit special.

 * If workspacesOnlyOnPrimary was true, we'd create one WorkspacesView, and
   additional montiors would be handled inside that WorkspacesView as
   "extra workspaces".

This caused numerous bugs as the two modes weren't consistently
implemented, and a lot of code was duplicated between all the modes.
Fix this by always creating WorkspaceViews, even if it only handles
one interface. We do this by having two different WorkspacesView-ish
classes: WorkspacesView handles the traditional combination of lots
of workspces, and a new ExtraWorkspaceView is in control of only one
workspace.