The ZipHelper interface is now more generic and allow single files to be added.
It supports adding uncompressed files (for PHP7+), which is required to have the mime detection magic work with ODS files.
Also fixed a few issues with the created ODS file (thanks to https://odf-validator.rhcloud.com/)
Heuristics to detect proper mime type for XLSX files expect to see
certain files at the beginning of the XLSX archive. The order in which
the XML files are added therefore matters.
Specifically, "[Content_Types].xml" should be added first, followed by the
files located in the "xl" folder (at least 1 file).
Added ODS writer
Refactored XLSX writer to abstract some pieces into an abstract multi-sheets writer
Created an abstract style helper
Moved shared components around
