Add DockMixin.position_dock and reset of expanded items

[Chilipp]

This method adds a new method to the DockMixin to set the position of the DockWidget. By default, this is implemented as

    def position_dock(self, main, *args, **kwargs):
        main.addDockWidget(self.dock_position, self.dock, *args, **kwargs)

and called in the to_dock method.

Furthermore, the DatasetTree now does not collapse the items anymore after the project as been updated.

• [x] Tests added (for all bug fixes or enhancements)
• [x] Tests passed (for all non-documentation changes)
• [x] Fully documented, including CHANGELOG.rst for all changes