Revise workspace next/prev

Revise workspace next/prev

See the issue #1798 (http://github.com/i3/i3/issues/1798).

+workspace_next+ as-is cycles through either numbered or named workspaces,
but when it reaches the last numbered/named workspace, it only looks for
named workspaces. This commit changes it: look for named workspaces after
exhausting numbered ones, but also for numbered ones after exhausting
named ones.

Also add a test case 528-workspace-next-prev.t (numbered workspaces and named
workspaces on 2 outputs) for testing this.