Tweak code formatting
API: Add top sellers endpoint

Part of #4055
fixed empty coupons select in reports page, fixes #4549
fixed the indentation in includes/admin/reports/class-wc-report-coupon-usage.php
fixed some coding standards in includes/admin/reports/class-wc-report-coupon-usage.php
fixed the order notes tooltip
fixed the indentation in includes/admin/post-types/class-wc-admin-cpt-shop_order.php
fixed some coding standards in includes/admin/post-types/class-wc-admin-cpt-shop_order.php
added a placeholder to select in category widget on reports page
fixed the indentation in includes/admin/reports/class-wc-report-sales-by-category.php
fixed some coding standards in includes/admin/reports/class-wc-report-sales-by-category.php
Merge pull request #4555 from claudiosmweb/added_placeholder_for_category_reports

Fixed untranslatable string in reports by category
Merge pull request #4552 from claudiosmweb/fix_4549

Hotfix for #4549
Merge pull request #4550 from maxrice/rest-api-top-sellers-endpoint

API: Add top sellers endpoint
Merge pull request #4554 from claudiosmweb/fix_shop_order_columns

Fix warning in order list screen