Change project files output to not include a prefix '/' in filename

It make sense to not put the initial '/' in the filename as the name
is relative to the top directory it belongs to.

By removing the the initial '/' in the filename we can re-introduce the
variable

core.project_files

to stay compatible with standard Lite. The project_files variable will
always point to the files table of the first entry of
core.project_directories.

data/core/statusview.lua

@@ -120,7 +120,7 @@ function StatusView:get_items()
     style.icon_font, "g",
     style.font, style.dim, self.separator2,
     #core.docs, style.text, " / ",
-    core.project_files_number(), " files"
+    #core.project_files, " files"
   }
 end