Use ListModels

[leolost2605]

This builds on top of #523 and allows us to provide a listmodel of objects and bind them to the itemgroup similar to how we bind listmodels to listboxes.

This has a few advantages:

• remove most of the mess in itemmanager (lots of type checking, custom layouting, work arounds)
• simplify the item handling. For example on main when a workspace is removed the workspace system receives that signal -> removes it from its internal list -> calls remove() on the workspace -> that emits the removed signal on the workspace -> the workspace icon group is connected to that and emits removed() on the baseitem -> the itemmanager had connected to that and starts removing the item. That involves 5 classes. With this branch it goes workspace system receives that signal -> removes the workspace from its internal list which is now a listmodel -> the rest is handled automatically via the items changed signal of the listmodel in the item group. That involves 2 classes.
• allows to add new sections to the dock much more easily

[leolost2605]

This should be ready for review now. You can look at the individual commits for easier review. The first one is the same as #523.