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.h

@@ -111,6 +111,7 @@ public:
 
     bool hasTotp() const;
     bool isExpired() const;
+    bool isAttributeReference(const QString& key) const;
     bool isAttributeReferenceOf(const QString& key, const QUuid& uuid) const;
     void replaceReferencesWithValues(const Entry* other);
     bool hasReferences() const;