Extend the index file format to store attributes of the index itself

[RKrahl]

At the moment, the index file is just a list of index items with their attributes. But it turns out that we need to maintain attributes of the index as a whole, see #67.

Define a new version 1.0 of the index file format (all legacy index files are retroactively denominated as version 0.1, for the variant created by photoidx 0.1 through 0.3 or as version 0.4 for the variant created by photoidx 0.4 through 0.10.*). Add another YAML document as header in front of the list of items. This should be a mapping with attributes of the index. The preliminary list of attributes includes:

• Version: version of the index file format,
• Date: last modification of the index file,
• Comment: a free text comment field,
• TimeZone: always None for the time being, but will get a meaning with Support time zone information in the createDate attribute in the index #67.