Global schedule view
As requested on GitHub: https://github.com/mhoeher/opentodolist/issues/68
It would be great to have a single view where scheduled items from any library would be shown. For this, the following probably needs to be done:
- Refine the business logic to allow query items from different libraries.
- Add meta data (specifically, a color) to libraries, so we can somehow clearly visualise from which library an item in such a global view comes.
- Implement the required GUI to display all scheduled items.
- To allow the user to create new items directly from that page, refine the item creation dialogs to allow selecting the target library to create new items in (for top level items - for todos, this is implicitly given via the todo list).