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

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

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

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

'yii\mail\MessageInterface' file related methods interface adjusted to use options, embed file methods added.

Method 'MailerInterface::createMessage()' added.

'SwiftMailer' recomposed into extension.

Email message render functionality added.

Methods 'addText' and 'addHtml' added to \yii\email\BaseMessage.

SwiftMailer applied as email solution.