Simply the combobox's internal state handling of the open-state

[haukesomm]

This PR simplifies the combobox's internal state handling in order to fix an issue related to its open-state and make it more robust in general:

• The open state of the combobox is no longer managed inside the component itself but instead fully controlled by the OpenClose mixin, making it more robust
• A regression test for an issue resulting in infinite opening and closing of the dropdown is added
• The component demo is restructured to allow more complex testing in the future: It is now split in two parts - the public demo and internal test drives. Some parts of the demo are moved from the public example to the internal testdrive.