First patch for gnucash

[Stefan Koch]

I think I am ready to propose my first patch for gnucash.  The content
is full test coverage (almost, see commit message) of the
libgnucash/engine/qofid.cpp file.  I figured this was a nice small
commit for me to trial the process on, and for y'all to provide me
with feedback for all the things I did not know.  The branch also has
three other commits that cleanup some stuff in the qofid.cpp code.  I
know the is bold for a hopeful first timer, but please take a look at
them before passing judgment and feedback.

I have followed the instructions and made a pull request here:
https://github.com/Gnucash/gnucash/pull/2169

Since this is my first time, I thought I would ask here if I am doing
this right, and how to proceed (other than awaiting github emails for
the review/merge workflow).

I am also looking for what to work on next.   I very much like your
long term plans for the code on this page:
https://wiki.gnucash.org/wiki/Roadmap.  But I know I am really new to
this, so adding more testing probably makes more sense before diving
into those items.  I will pick something myself and start working, but
if anyone has good ideas, I can look there.