Excel2007: When using disk caching, do not read all the files into memory.

The potential gain in memory usage offered by disk caching was
annihilated by the fact that the generated content was read into memory
before being added to the archive.
Excel2007: do not uselessly read image data when zipping.

When we have a file name for the image being added to the ZIP file,
there is no need to read it into memory.
Excel2007: Use a local variable for the current sheet.

This is to improve readability.
Excel2007: Introduce WriterPart::createXMLWriter()

This reduces code duplication among the various parts.
Excel2007: Handle temporary files at the WriterPart level.

XMLWriter now takes the temporary file name as an argument, allowing us
to make a better management of the temporary files though the tracker
object. This allows us to manage the lifetime of the various temporary
files more easily.