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)

tests/TestGroup.cpp

@@ -1181,3 +1181,29 @@ void TestGroup::testApplyGroupIconRecursively()
     QVERIFY(subsubgroup->iconNumber() == iconForGroups);
     QVERIFY(subsubgroupEntry->iconNumber() == iconForEntries);
 }
+
+void TestGroup::testUsernamesRecursive()
+{
+    Database* database = new Database();
+
+    // Create a subgroup
+    Group* subgroup = new Group();
+    subgroup->setName("Subgroup");
+    subgroup->setParent(database->rootGroup());
+
+    // Generate entries in the root group and the subgroup
+    Entry* rootGroupEntry = database->rootGroup()->addEntryWithPath("Root group entry");
+    rootGroupEntry->setUsername("Name1");
+
+    Entry* subgroupEntry = subgroup->addEntryWithPath("Subgroup entry");
+    subgroupEntry->setUsername("Name2");
+
+    Entry* subgroupEntryReusingUsername = subgroup->addEntryWithPath("Another subgroup entry");
+    subgroupEntryReusingUsername->setUsername("Name2");
+
+    QList<QString> usernames = database->rootGroup()->usernamesRecursive();
+    QCOMPARE(usernames.size(), 2);
+    QVERIFY(usernames.contains("Name1"));
+    QVERIFY(usernames.contains("Name2"));
+    QVERIFY(usernames.indexOf("Name2") < usernames.indexOf("Name1"));
+}