Main motivation for this change is that it's unhandy to have the class
definition in workspace.h and client.h while the implementation is in
events.cpp although nothing in events.cpp uses it directly.

By getting it out of workspace.h we get the header a little bit smaller
which should improve compile time given that it's included almost
everywhere.

In events.cpp the enum usage is changed to NETWinInfo as that's the class
where they are defined.

RootInfo does no longer hold a workspace pointer. Where it's needed it
uses the singleton accessor of Workspace.

REVIEW: 110199