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/plugins/projectsearch.lua

@@ -52,7 +52,7 @@ function ResultsView:begin_search(text, fn)
     local i = 1
     for dir_name, file in core.get_project_files() do
       if file.type == "file" then
-        find_all_matches_in_file(self.results, dir_name .. file.filename, fn)
+        find_all_matches_in_file(self.results, dir_name .. PATHSEP .. file.filename, fn)
       end
       self.last_file_idx = i
       i = i + 1