Article count shown in dictionary info is incorrect for multi-volume dicitonaries compiled with aardtools 0.9

[itkach]

In aardtools 0.9 the way volumes are generated has changed and the meaning of article_count metadata property changed to mean total number of articles in this volume, not in the whole dictionary. Application should be updated to sum up article counts for all volumes to display accurate value in dictionary info for dictionaries generated with aardtools > 0.9.

[itkach]

This turned out harder than I thought: metadata is assumed to be the same across all volumes and is cached per dictionary. Caching is essential since bz2 decompression is prohibitively slow on Android, even on latest devices.