Currently the spreadsheet dynamically creates the FilterMenu for a range, that is, it's not created until someone clicks the filter dropdown, and it's destroyed if they switched tabs.

There is no event that is fired when these things happen, which makes it hard to attach to, and customize, the FilterMenu without setting up some crazy listening code (setTimeout, etc.). 

It would be great if either (or both:
- You could specify a template for the filter menu.
- You could listen for a "FilterMenu Initialized" event and capture the menu there and customize it.