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.h

@@ -48,6 +48,7 @@ private slots:
     void testChildrenSort();
     void testHierarchy();
     void testApplyGroupIconRecursively();
+    void testUsernamesRecursive();
 };
 
 #endif // KEEPASSX_TESTGROUP_H