This renames the very generic-named icons folder to better match
what it actually contains, File Icons.
App Icons are moved to their own folder as they are not file icons.
This matches the new structure in Xcode better too.
Pref icons folder is renamed to match other folder names and the icons
are renamed to have a simpler name.
This moves the button icons (PDF template images) all to one folder
so it is easier to find them in the sources and having them separate
does not make much sense anyway, given that these icons were not
exclusively used by the menu bar icon.
