"yii\swiftmailer\Mailer" transport setup has been advanced to support constructor arguments and plugins.

also removed superflous minimum-stability definition

This reverts commit 7d6d01b4.

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.