WooCommerce Currency Converter Widget 2.2.3

* Feature - Added setting for defining the rates refreshing period.
* Tweak - Validate the App ID on settings save.
* Tweak - Tested compatibility up to WC 7.5.
* Tweak - Tested compatibility with WP 6.2.
* Dev - Log errors from the Open Exchange Rates API requests.
* Dev - Initialize the plugin when the main file is loaded.
* Dev - Removed trailing underscore from the filter hook `wc_currency_converter_settings_api_form_fields`.
* Dev - Dropped deprecated code from versions 1.x.
* Dev - Updated the minimum requirements to WC 3.7, WP 4.9, and PHP 5.6

* Tweak - Improved the widget CSS styles when using the 'Buttons' option.
* Tweak - Use the WC locale info to assign the currency to a geolocated country.
* Tweak - Tested compatibility with WC 7.3.
* Fix - Fixed duplicated currency when using the asterisk (*) in the default currency.
* Dev - Refactored widget class by extending the class `WC_Widget`.
* Dev - Deprecated class `WooCommerce_Widget_Currency_Converter`.

* Tweak - Moved the plugin settings to an integration page.
* Tweak - Added links to the plugin settings, the documentation page, and to get support, in the plugin list.
* Tweak - Declare compatibility with WooCommerce High-Performance Order Storage (HPOS).
* Tweak - Tested compatibility up to WC 7.2.
* Tweak - Tested compatibility with WP 6.1.
* Tweak - Updated plugin author.
* Dev - Updated the library `money.js` to version 0.2.0.
* Dev - Use the library `accounting.js` included in the WC core.
* Dev - Implement the singleton pattern in the main plugin class