Merged the changes made in 2.1.7b.
Added a type-hinting in a method.
Moved the sort method into the page base class from the property.
- Added a basic usage example in the demo plugin.
- Changed the page-heading tabs visibility to be enabled only when multiple sub pages are added.
- Made it use a default menu and screen icons if they are not set.
- Made setRootMenuPage() to support dash icons.
- Added an example to use a dash icon in the demo plugin.