PluginDirectory - avoid BusyDialog if not called by main thread

[FernetMenta]

follow up from #9778

[Montellese]

Ideally there shouldn't be any GUI related code in the VFS part (no dialogs or display related stuff like the "All albums" nodes etc.). That logic should be part of CGUIMediaWindow or any of its derived classes. But obviously that's a major task and out of scope for this fix.

[FernetMenta]

updated
jenkins build this please

[FernetMenta]

Android build failure unrelated

[BmdOnline]

Hi,
like I said in kodi forum (http://forum.kodi.tv/showthread.php?tid=269814&pid=2345031#pid2345031), some plugins aren't working since Milhouse 0522 build.

For example, with Youtube 5.2.1 plugin, I have this message in kodi.log

23:09:36 12917.675781 T:1962225664 ERROR: GetDirectory - Error getting plugin://plugin.video.youtube/channel/UCMkOKU7usWmqcpLDcWG9PtQ/
23:09:36 12917.719727 T:1962225664 ERROR: CGUIMediaWindow::GetDirectory(plugin://plugin.video.youtube/channel/UCMkOKU7usWmqcpLDcWG9PtQ/) failed

And, with SuprFavourites 1.0.43 plugin, I have same kind of error :

23:12:00 13061.955078 T:1962225664 ERROR: GetDirectory - Error getting plugin://plugin.program.super.favourites/?fanart=%2Fstorage%2F.kodi%2Faddons%2Fplugin.program.super.favourites%2Ffanart.j​pg&image=DefaultFolder.png&label=Explore%20Kodi%20favourites&mode=300
23:12:00 13061.968750 T:1962225664 ERROR: CGUIMediaWindow::GetDirectory(plugin://plugin.program.super.favourites/?fanart=%2Fstorage%2F.kodi%2Faddons%2Fplugin.program.super.favourites%2Ffanart.j​pg&image=DefaultFolder.png&label=Explore%20Kodi%20favourites&mode=300) failed

Everything is OK with 0421 build.