Small mainthread check for add trackers
Fix utf-8 path problem on Windows.
Schedule new_tracker popup on mainthread
Merge pull request #432 from NielsZeilemaker/mainthread_check

Mainthread check
Remove the import of NATFirewall.
Merge pull request #434 from LipuFei/rm-import-natfirewall

Remove the import of NATFirewall.
Merge pull request #431 from LipuFei/fix-shutil-utf8

UTF-8 paths for shutil functions.