Improvements to phases and processors

Problem/Motivation

This issue mostly affects developers.

1. The "phases" of email processing are not that clear.
2. The requirements of what can be done in each phase are a bit muddled.
3. The email processors and hooks are a bit confusing and rigid.

Proposed resolution

1. Add constants for the phases on EmailInterface with clear descriptions.
2. Adjust the set of phases.
   • Init is to set processors and change parameters.
   • Build is before the "switching" of language/theme/account, must not trigger rendering in this phase.
   • PreRender is after the switching and before rendering (rarely needed).
   • PostRender after rendering
3. Change Email::addProcessor to accept a simple callable giving greater flexibility and simplicity.

Remaining tasks

User interface changes

API changes

• Most implementations of EmailProcessorInterface::preRender() should change to use EmailProcessorInterface::build(). However could will mostly still work if no change is made.
• Removed hook_mailer_pre_build(), hook_mailer_pre_render(), hook_mailer_post_render(). Instead use hook_mailer_init() and call $email->processor() registering your callable.
• EmailInterface::addProcessor() changed to support the above. It's more flexible allowing any weight, any callable.
• New function EmailProcessorInterface::initialize() changed, but classes derived from EmailProcessorBase are unaffected so it's unlikely to affect anyone.

Data model changes