Expand EntityViewMode with methods

Problem/Motivation

Part of meta-issue #2016679: [Meta] Expand Entity Type interfaces to provide methods.

See the detailed explanations there and look at the issues that already have patches or were commited.

Add get*, set* and additional methods as it makes sense to replace the public properties (e.g. isSomething() and something())

Remaining tasks

Contributor tasks needed

Task	Novice task?	Contributor instructions	Complete?
Reroll the patch if it no longer applies.		Instructions
Add automated tests		Small tests to make sure setters work properly. Instructions
Draft a change record for the API changes		Instructions
Review patch to ensure that it fixes the issue, stays within scope, is properly documented, and follows coding standards		Instructions

User interface changes

NONE

API changes

EntityDisplayModeInterface now uses setTargetEntityTypeId() and getTargetEntityTypeId() to get and set its $targetEntityType property.
The EntityDisplayModeBase class implements the EntityDisplayModeInterface class, and these changes are reflected there too.