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

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

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

Method 'MailerInterface::createMessage()' added.

'swiftmailer' extension unit tests fixed.

'SwiftMailer' recomposed into extension.

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

SwiftMailer applied as email solution.