[7.x] Add theme to mailable properties

[basselhossam]

this allows the user to change the theme in the runtime and reflect it in queue because when the object is serialize for queue $this->theme would be gone if it's not preserved in the object properties

for example if my __construct looks like this

public function __construct($user)
{
     $this->theme = 'default-' . $user->user_lang;
}

I'm send an email to user and based on the user language I would like to set the theme

so by adding $theme to the properties it will be serialized and can be retrieved in the queue correctly