Fix tab completion

Fixes #813

This has the "quirk" of not updating names that are already there, which
means that display names might not always be used, instead the users
lowercase name might just be there and stick

src/common/UsernameSet.cpp

@@ -59,7 +59,7 @@ void UsernameSet::insertPrefix(const QString &value)
 {
     auto &string = this->firstKeyForPrefix[Prefix(value)];
 
-    if (string.isNull() || value < string)
+    if (string.isNull() || value.compare(string, Qt::CaseInsensitive) < 0)
         string = value;
 }