Closing a tab doesn't update the tab count immediately

[jodizzle]

Example:

• Five tabs open in a window, the addon icon shows the number 5, as expected.
• Close a tab so that there are now four tabs in total. The addon icon still shows the number 5.
• Close another tab so that there are now three tabs in total. The addon icon now shows the number 4.

Closing more tabs will continue to reduce the number, but it will remain off by one. Un-focusing then re-focusing the window, pinning/un-pinning a tab, or any other behavior that there's a listener for will re-align the counts.

I think this has something to do with the onRemoved listener triggering too quickly, or possibly the queryNumTabs function counting a tab that's in the process of being closed. I'm not sure what the best fix is.