Fix plenty of logic errors in the SubtreeModel

These methods were so wrong that it was not even funny. The *weew* used; how
come that this was not caught before?
SubtreeModel: reimplement forwarding for dropMimeData

The QAbstractProxyModel doesn't handle this for us, so message copying and
moving via drag and drop was broken in the previous version.
Use ImapAccess even in the desktop version
Use one common code path for model initialization on both desktop and QML versions

REVIEW: 115438