Use data from other entries and collections in a custom preview

**Is your feature request related to a problem? Please describe.**
Getting data in the custom preview is limited to the current `entry`, so it's not possible to make previews that rely on multiple data sources.

**Describe the solution you'd like**
I'd like to fetch data from other entries and collections. Preferably in a data object just like `entry`. It would be even better if collections would get their own property.

**Describe alternatives you've considered**
I thought about fetching the data just like the relation widget does, but to be honest: I couldn't wrap my head around it. Currently I fake the data by adding generic bits that look like the real thing, but in the end that can also be confusing.