* ABOUT-NLS: Add missing file to fix the build.

* src/aboutdialog.cc: Translate all user-visible strings except for those instances where positional parameter substitution is required for proper results.  I'll deal with those ones later.
* src/controller.cc: ditto
* src/filebuffer.cc: ditto
* src/filetree.cc: ditto
* src/main.cc: ditto
* src/mainwindow.cc: ditto
* src/pcreshell.cc: ditto
* src/prefdialog.cc: ditto
* src/statusline.cc: ditto.

* src/translation.h (N_): #define to gettext_noop(s) rather than simply (s).  This doesn't actually make a difference but I think the intent is clearer that way.