[FEATURE] Proper foreignTypes on all inline fields

This change adds:

* An InlineRelationFieldInterface with get/setForeignTypes
* `foreignTypes` (array) argument on `flux:field.inline`
* Transfer of `foreignTypes` to `Inline` component in `flux:field.inline`
* Touch-up added variable type declaration in `flux:field.inline.fal`
[TASK] Fix broken ErrorReporter test
[DOC] Declare vague mixed as array, Abstract Inline Component
[TASK] Cleanup by avoiding unnecessary FormComponent->add
Merge pull request #510 from NamelessCoder/inlineforeigntypes

[FEATURE] Proper foreignTypes on all inline fields