Add WSD, remove movement restriction.

It is now possible to move anywhere (and thus create workspaces at any
position). The workspace dialog displays where the view currently is,
and shows all workspaces nearby that are persistent (the 'wsdradius'
variable determines the distance of the "map").

README update.
Merge branch 'wsmenu'.

Allows toggling a workspace dialog that displays a "map" of all open
workspaces nearby.