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.

Related to b3b5e36d.

not need to suggest packages in core. They are required by extension
composer.json anyway.

- rename FileHelper::mkdir for API consistency
- changed default permission for directories to 775 instead of 777
- added some properties to classes that deal with files to allow control
  over directory permissions.