Implement sorting location items by the number of visits

Implement sorting location items by the number of visits

For the moment, a define in the code decides whether items
are sorted based on when they were added or how often
a page was visited. The 'visits' property and respective
database column is unused (we keep it for compatibility).
It turns out it's enough to store that information in
the tree model.

The visit based sorting is not enabled because it is
simply delaying startup incredibly. It will have to be
decided whether to introduce a preference, or always use
a visited based sorting once the startup delay is fixed.

History items are also deleted from the tree model now
if they are too old, according to the preference.