Improve the client-side code that keeps unread counts up to date in the browser.

When all entries from a feed or folder have been loaded, unread entries actually received for each feed are counted and the client-side unread count is updated. This ensures that the unread is (more or less) correct when pagination finishes. Unfortunately it may be wrong until the last page of entries has been loaded; this is unavoidable, the client simply does not know how many unread entries are in not-yet-received pages.

No action is done to notify the server of any error in unread counts. Scheduled jobs are supposed to run and fix this information as needed.

Previously a limited functionality was implemented: when loading a folder's entries, feeds which had returned no entries got their unread count set to zero. This commit implements a superset of that: now it works both when reding a feed and when reading a folder, and also the unread count is corrected for all feeds, not just for those that have no unread entries.

Also a bug has been fixed: the old implementation counted entries without regard to their read/unread state; this meant that if "show read" had been selected, the unread count could very well be wrong because it included read entries.
When all entries have finished loading, update the total unread count in the favicon.

In 7adc056d72fb4b3ef331aa79dd29ceba5fe22861 readSvc was changed so that unread counts in feeds were updated when entries pagination was finished. However the total unread count in the favicon was not updated as well, which could cause a discrepancy between the count reported in the sidebar (and window title) and the count reported in the favicon.

To fix this, the change in the unread count in readSvc is not done accessing the unread_count attribute in feed objects directly, but rather invoking a method in changeUnreadCountSvc. This method updates the unread_count attribute in feed objects, and it also takes care of invoking favicoSvc to update the favicon count.

Generally, changes in the unread count of feeds should be done through changeUnreadCountSvc, so that any extra actions (like updating the favicon) are handled transparently.