remove setting attachment of unknown original size to max image size

This was originally set to {1,1}, later maxImageSize. By not being Zero a custom subview for an image would get a chance to be loaded lazily, e.g. with DTLazyImageView. This behavior was changed since now a subview is also requested for a subview with size zero.
request subview for attachment, even if frame size is zero

This way a lazily loaded image view gets a chance.
Refactored relayout after lazy image loading into DTAttributedTextView.

Without an extra setNeedsLayout an lazy image would not show up since we switched to a tiled layer for the content view and inside scrollviews the scroll view is responsible for the lay outing of just the visible custom subviews.

This reduces the code to put into lazyImageView:didChangeImageSize:. I also change the sizing behavior for zero size attachments. They still request a subview now.

Fixes #368
Added synchronized getter for child nodes

related to #335