Ensure project rescan thread is terminated

When changing a project we need to ensure that the old threads
are no longer run.

data/core/init.lua

@@ -369,13 +369,14 @@ end
 local function add_dir_scan_thread(dir)
   core.add_thread(function()
     while true do
+      print("DEBUG: running rescan on", dir.name)
       local has_changes = rescan_project_subdir(dir, "")
       if has_changes then
         core.redraw = true -- we run without an event, from a thread
       end
       coroutine.yield(5)
     end
-  end)
+  end, dir)
 end