Add "Schema" tab

[Reinmar]

Schema contains a lot of information provided by the features. Without checking the code of these plugins, it's hard to understand what's the schema configuration.

We should provide a way to browse the schema, perhaps query it, log the queries that are being made by features, etc.

Related: #40

[pomek]

An idea:

• Display names of schema definitions the same way as commands (in a list).
  • After clicking on a single item in the right column, display its details.
• Based on the selection - gray out definitions that aren't allowed in the current context.

Problems:

• Where should we display attribute properties?

[pomek]

Another idea: toggling schema properties (e.g. allowIn, etc.).

But as for now, moving to "Blocked" as we are not sure what and how to show it.

[oleq]

PoC https://github.com/ckeditor/ckeditor5-inspector/compare/i/41?expand=1