Fix make makedistcheck and re-organize icon paths.

Fix make makedistcheck and re-organize icon paths.

This change does several things:

* renamed psppicon.png to pspp.png - The word "icon" is redundant
  (it's also a misuse of English).

* Changed the install target of the existing 16x16 pictograms out of the apps
  directory. Instead, they now go in a directory of their own called "pspp".
  This is more in line with what other applications seem to do.  It also makes
  it a lot easier to deal uninstall them, since we can be reasonably sure that
  everything in the "pspp" directory was put there by us.

* Ensures that make distcheck passes.