enh: add a UI function to list task files

[Selene ToyKeeper]

I find I often want to see a list of my task files, so I can quickly filter to a different file or add a task. Currently, I use a kludge to do this, but a real function would be handy.

The kludge I use is:
  - Add a placeholder task called '...' to the top of each file.
  - Type '/...' whenever I want to see the file list.

Instead, it should be a matter of pressing a single key.

Fixing bug 482491 would be a helpful first step to implementing this.