Improve statistics

[tharders]

Currently the statistics show the number of entries for each content type and a total number.
For each content type we should add:

• Number of written (changed/new) files
• Number of skipped files (only when > 0)
• Number of invalid files (only when > 0)

Next to the total number of entries at the end the number of deleted files would be great.

We also have to check the handling of the locales in the statistics.
The statistics are printed only for the default locale.
But what if an entry is skipped for the default locale?
Printing the statistics for every locale would be too much when we have a long list of locales.
Maybe we could only print something when the values differ in a locale?