Improve CSV export and import capability

* Fixes #3541
* CSV export now includes TOTP settings, Entry Icon (database icon number only), Modified Time, and Created Time.
* CSV import properly understands time in ISO 8601 format and Unix Timestamp.
* CSV import will set the TOTP settings and entry icon based on the chosen column.

src/core/Entry.h

@@ -106,6 +106,7 @@ public:
     QString notes() const;
     QString attribute(const QString& key) const;
     QString totp() const;
+    QString totpSettingsString() const;
     QSharedPointer<Totp::Settings> totpSettings() const;
     int size() const;