Add settings page
- Remove theme button in favour of changing it from
a new settings page.
- Model for setting is a simple key/value pair but via
an interface if we need some more data in it.
- Add store concept and facilities around it.
- For now this settings store is dummy as it doesn't
yet integration into any permanent store.
- Change Manage item as settings page took the cog shape.
- Fixes #1477