gui: Add debug tab to settings (ref #2644)

Just because there are a ton of people struggling to set env vars.
Perhaps this should live in advanced settings, and perhaps we should have a button to view the log.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4604
LGTM: calmh, imsodin

lib/fs/debug.go

@@ -14,9 +14,9 @@ import (
 )
 
 var (
-	l = logger.DefaultLogger.NewFacility("filesystem", "Filesystem access")
+	l = logger.DefaultLogger.NewFacility("fs", "Filesystem access")
 )
 
 func init() {
-	l.SetDebug("filesystem", strings.Contains(os.Getenv("STTRACE"), "filesystem") || os.Getenv("STTRACE") == "all")
+	l.SetDebug("fs", strings.Contains(os.Getenv("STTRACE"), "fs") || os.Getenv("STTRACE") == "all")
 }