Views now maintain a reference to a launch context which, as a last
resort, is populated at map time with a context associated with its pid.
This opens the possibility of populating it before map via another
source, e.g. xdga-tokens or configuration.
This removes the need to rename the pid_workspaces when a workspace
is renamed.
It also opens the possibility of tracking other node types. Tracking
containers would allow application to be placed correctly in the
container tree even if the user has moved their focus elsewhere since
it was launched.
