Fix dirmonitor sorting issues (#1599)

* Use `PATHSEP` in path-related functions

* Don't stop on digits when getting the common part in `system.path_compare`

* Avoid sorting multiple times in `dirwatch.get_directory_files`

This also fixes the timeout detection in `recurse_pred`.

data/plugins/treeview.lua

@@ -24,7 +24,7 @@ local tooltip_alpha_rate = 1
 
 local function get_depth(filename)
   local n = 1
-  for sep in filename:gmatch("[\\/]") do
+  for _ in filename:gmatch(PATHSEP) do
     n = n + 1
   end
   return n