more expected behavior when chaining methods using
Mailser::compose()

fixes #2723

issue #2852

allows opening email for view in mail program directly

Method 'yii\mail\MailerInterface::compose()' reworked allowing rendering message body.

- 'body()' renamed to 'renderBody()'
- new 'body()' method introduced

Method 'yii\mail\BaseMailer::compose()' updated, allowing composition of the complex methods like 'renderText' and 'body'.

- setter methods renamed to have pure name
- method 'createMessage' renamed to 'compose'

MessageInterface::render() replaced by MessageInterface::renderHtml() and MessageInterface::renderText()

Interface 'ViewContextInterface' applied to BaseMailer.

Method 'MailerInterface::createMessage()' added.

'SwiftMailer' recomposed into extension.

Email message render functionality added.

SwiftMailer applied as email solution.