Fixes a couple code reviews.
More things can just use @inheritdoc.
Add some inline docs as to why show() was needed.
d.o #2281491, starting on dependency injection.
d.o #2281491, Now using injected entityManager
d.o #2281491, Now using injected entityManager
d.o #2281491, Refactoring on the suggestion of Dave Reid
d.o #2281491, Adding a todo
d.o #2281491, Adding a property declaration.
Merge pull request #32 from stevector/2281491-more-dependency-injection

Adding dependency injection to EntityEmbedDefaultDisplay.
Merge branch '8.x-1.x' into embed-display-review