workbench: add get_active_workspace() helper.
workbench: use proxy GActionGroup to dispatch actions to workspaces.

Previously, we would have had to proxy all global commands via the
workspace interface. This is pretty tedious and britle. So instead,
you can now just define the actions in the workspace and the proxy
will hook them up to "workbench.<action>" if they are also defined
in gb-workbench-actions.