Issue 10: Detect need for reindex when upgrading filters

There are cases when a filter is improved enough during a version upgrade, in a way that would make it worth reindexing the files it handles.

This is not a simple issue because it depends both on the type and amount of filter improvements and on user preference.

In order to make any progress in automating this, the filter version number should be stored in the index, for each document.

A lighter solution would be to propose an easy way to purge all documents indexed by a given filters. The operation could then be recommended by the release notes and performed by the user.

The current recoll version has code for reindexing failed files if some filter or aux program may have changed. This dies not handle docs for which indexing succeeded but which could be improved by a reindex. A command line option to recollindex seems appropriate for this (recollindex --reindex mimetype).

This is adequately solved by recollindex -r -Z -p *.xxx