CommandManager: add new func
NavigationList: add new func
widget: gb_widget_get_workbench() should return GbWorkbench
workbench: create document manager for GbWorkbench
grid: use assigned document manager, not default
DocumentManager: remove get_default() helper

Too easy to accidentally use this, when what we really want is to get the
document manager for the workbench (which will soon have a Project attached
to it).
workbench: use our own document manager
workbench: general cleanup
EditorWorkspace: use document-manager from Workbench