Don't show notifications for tabs that are not the current one

[terrycojones]

Some web pages reload themselves automatically, causing the popup notification to be shown repeatedly, even if the tab they're in isn't currently being shown. It's pretty annoying / useless. So only show notifications if the tab that's loading is the one that's being looked at. This will also stop a flood of notifications when restarting the browser if it's restoring state and many tabs have tags on them.