gEDA-dev: add-on dir name for pcb

[DJ Delorie]

Ok, I've got a patch to have pcb dynamically load anything it finds in
a directory, but I'm a little stumped as to what to call the
directory.

The patch looks in ~/.pcb/hids/ at the moment, but my first test case
defined a new user-defined action, not a hid at all.  In theory, we
can define actions, flags, attributes, and/or hids, in those files.

So the question is, what to name that subdirectory.  Keep it "hids"
and just document the other uses?  Or name it something else?

(yes, it will look in $prefix too)