In the workspace applet, this now uses `Workspace` in the front-end code
instead of a tuple with unnamed fields. Handling of scrolling is also
moved to the frontend, which uses less code and seems more natural. It
would be good to have a helper in libcosmic for this. It also changes
`ObjectId` to `ExtWorkspaceHandleV1`, which is a little simpler and I
see no reason here to avoid the more strongly typed object.
At some point we may want a shared subscription for workspaces in
multiple applets. As well as a higher-level abstraction for screen
capture.
