Handle indirect stream filter/decode parameters

QPDFWriter was trying to make /Filter and /DecodeParms direct in all
cases, but there are some cases where /DecodeParms may refer to a
stream, which can't be direct. QPDFWriter doesn't actually need
/DecodeParms to be direct in that case because it won't be able to
filter the stream. Until we can handle this type of stream, just don't
make /Filter and /DecodeParms direct if we can't filter the stream
anyway.

Fixes #34
Example: fast split into single pages

This is faster than using qpdf --pages to do it.
Ignore external-libs directory

Used during creation of Windows releases
Ignore some files created by MSVC
Windows build (msvc): target Windows 5.0.1 (XP)

Without this, qpdf executables work only on Vista or newer.

Fixes #35
Prepare 5.1.2 release