Added: the ability to set the target page slug in the `addInPageTabs()` method so that the page slug key can be omitted for the next passing tab arrays.
Added: the ability to set the target page and tab slugs in the `addSettingSections()` method so that the page and tab slug keys can be omitted for the next passing section arrays.
Made the addSettingSection() method support the target page slug and the target tab slug.
Removed redundant lines due to the newly supported target parameter in the demo plugin.
Removed redundant lines.
* Added: the ability to set the target section ID in the `addSettingFields()` method so that the `section_id` key can be omitted for the next passing field arrays.
- Removed the existing parameters of `addInPageTab()` method and changed it to one parameter that accept a in-page tab array or the target page slug.
- Removed redundant liens in the `addInPageTabs{)` method.
Refactored some methods.
Refactored a method.