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/Config.cpp

@@ -179,7 +179,6 @@ void Config::init(const QString& fileName)
     m_defaults.insert("SearchLimitGroup", false);
     m_defaults.insert("MinimizeOnCopy", false);
     m_defaults.insert("MinimizeOnOpenUrl", false);
-    m_defaults.insert("UseGroupIconOnEntryCreation", false);
     m_defaults.insert("AutoTypeEntryTitleMatch", true);
     m_defaults.insert("AutoTypeEntryURLMatch", true);
     m_defaults.insert("AutoTypeDelay", 25);