Adds ContentValue and ContentValueFilter to replace ValueBlockData so that it can be used in attributes, etc.
Adds action to BlockData.Text and BlockData.Value.
Adds ContentValue.StaticText and ContentValue.RawHtml to replace ValueBlockData.Text and ValueBlockData.Html.
Removes unused classes.
Renames ContentValue.StaticText to Text for consistency.
Changes AbstractBlock#data to be required and adds placeholder to #text.
Adds ContentStream.Mix to replace PinContentStream.