This commit refactors the ZenTabUnloader class to add support for unloading inactive tabs. It introduces a new class called ZenTabsIntervalUnloader, which periodically checks for inactive tabs and discards them if they meet certain criteria. The criteria include being inactive for a specified timeout period and not meeting any exclusion URLs.
The commit also adds a new class called ZenTabsObserver, which listens to various tab events and notifies the ZenTabUnloader when a tab's activity changes.
These changes improve the efficiency of tab management and help reduce resource usage by unloading tabs that are not actively being used.
