Action to close all hidden buffers

[dbarnett]

As I'm editing, I frequently get a long list of open buffers and gradually lose the ability to use the buffers list effectively.

There's the Alt-C action to close all buffers, but most of the time I have a bunch of files open in splits I want to preserve. It would be nice to have an action to just close the buffers that aren't visible.

Alternatively, if the buffers list had some visual indication of whether a buffer was visible, it would be pretty easy for me to go through and zap the hidden ones, or just ignore them.