Autocomplete usernames based on most frequent in database

* Fixes #3126
* Limit autocompletion to the top ten used usernames
- Load common usernames when database is opened
- Transition from QLineEdit to QComboBox for usernames
- Dropdown menu of the combobox lets user choose a common username
- Common usernames are autocompleted via inline completion
- Common usernames are sorted by frequency (first) and name (second)

src/core/Entry.cpp

@@ -341,6 +341,11 @@ bool Entry::isExpired() const
     return m_data.timeInfo.expires() && m_data.timeInfo.expiryTime() < Clock::currentDateTimeUtc();
 }
 
+bool Entry::isAttributeReference(const QString& key) const
+{
+    return m_attributes->isReference(key);
+}
+
 bool Entry::isAttributeReferenceOf(const QString& key, const QUuid& uuid) const
 {
     if (!m_attributes->isReference(key)) {