Introduce a new field in items generated by TreeView:each_item()
to point "dir" to the toplevel directory entry.
In this was we can simplify the code and know if the toplevel
directory is files limited.
Since the directory monitoring is now basically working we remove the
project scan thread periodically scanning the project directory.
Each project's directory is scanned only once at the beginning when
calling the function `core.add_project_directory` and is updated
incrementally when directory change events are treated.
The config variable `project_scan_rate` is removed as well as the
function `core.reschedule_project_scan`.
Francesco Abbate