Expressions: New not function, extend logic behind component

[olemartinorg]

This issue is fairly easy to implement, and could be implemented before the first release of #540. Slightly discussed on Slack between me and @ivarne, we decided on implementing two additional features of the expressions engine:

1. A new not function. Accepts one boolean (or null) as input, returns !input.
2. Extend the logic in component to return null if the found component is hidden. This allows for some limited chaining, and makes the component functionally more different from the dataModel lookup (which won't have the same logic).