Rename EscherRecordHolder to OfficeArtContent

While the class does indeed hold EscherRecords, due to recent refactoring it is much more structured now than it was before. The contents of the class now closely resemble the OfficeArtContent structure referenced in the MS-DOC spec. Naming the class after the specification structure makes it easier to find and understand.

git-svn-id: 13f79535-47bb-0310-9956-ffa450edef68
6 files changed