LibPDF: Move document-specific parsing functionality into its own class

The Parser class is now a generic PDF object parser, of which the new
DocumentParser class derives. DocumentParser now takes over all
functions relating to linearization, pages, xref and trailer handling.

This allows the use of multiple parsers in the same document's
context, which will be needed in order to handle PDF object streams.

Userland/Libraries/LibPDF/CMakeLists.txt

@@ -2,6 +2,7 @@ set(SOURCES
     ColorSpace.cpp
     CommonNames.cpp
     Document.cpp
+    DocumentParser.cpp
     Encoding.cpp
     Encryption.cpp
     Filter.cpp