[KPhotoAlbum] Testers wanted: Show date-range for tags and performance

[Johannes Zarl-Zierl]

Hi,

I've implemented a display of date ranges of tags. I.e. when browsing, you'll 
now see the time range of the images tagged with that specific tag.

While implementing this, I've also improved the performance of the 
Browser::OverviewPage::updateImageCount() method. This means that the overview 
page should now be displayed significantly faster.

I would be pleased to get comments on this (branch feature/categoryDates)...

Cheers,
  Johannes