Redesign ComposeView

The current multipleViews layout provides a highly non-standard UX to little benefit. We suggest a redesign that follows commonly held UI conventions for email compose views.

• change ComposeView from its current multipleViews implementation to use singleView layout
• expand text field for the email body (currently it's only a single line in singleView)
• add Send button
• add Cancel button
• add expanding recipient fields to make Cc/Bcc entries optional viewing (most emails don't use Cc/Bcc)
• attachment button
• delete attachments
• preview attachments
• check mail body for "Anhang", "attachment", "document" etc. and ask for attachment when missing