Refactoring Editor Lite's architecture

What does this MR do?

This is the first step of several smaller ones that intend to make the Editor Lite's architecture even simpler and easier extensible. The most important changes in this MR are related to supporting multiple instances of the same editor on one view:

• Single place for catching errors when creating an editor's instance
• An opportunity to extend instance on its creation in addition to the global editor's options. This should allow to have mix of differently configured instances (let's say mix read-only and normal editing instances) on one page with one root editor
• Now, when creating an instance, we return the instance so that we could use instance's API directly instead of routing through the editor. Doing so through the root editor might be impossible in a lot of cases of multi-editor views.

Screenshots

No visual changes are introduced. Hence no screenshots and no changelog entry have been provided.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done