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/Database.cpp

@@ -54,6 +54,7 @@ Database::Database()
 
     connect(m_metadata, SIGNAL(metadataModified()), this, SLOT(markAsModified()));
     connect(m_timer, SIGNAL(timeout()), SIGNAL(databaseModified()));
+    connect(this, SIGNAL(databaseSaved()), SLOT(updateCommonUsernames()));
 
     m_modified = false;
     m_emitModified = true;
@@ -149,6 +150,8 @@ bool Database::open(const QString& filePath, QSharedPointer<const CompositeKey>
     setFilePath(filePath);
     dbFile.close();
 
+    updateCommonUsernames();
+
     setInitialized(ok);
     markAsClean();
 
@@ -525,6 +528,17 @@ void Database::addDeletedObject(const QUuid& uuid)
     addDeletedObject(delObj);
 }
 
+QList<QString> Database::commonUsernames()
+{
+    return m_commonUsernames;
+}
+
+void Database::updateCommonUsernames(int topN)
+{
+    m_commonUsernames.clear();
+    m_commonUsernames.append(rootGroup()->usernamesRecursive(topN));
+}
+
 const QUuid& Database::cipher() const
 {
     return m_data.cipher;