New Widgets

[kurawlefaraaz]

For new widgets only:

Title: Merge NumberedText, DynamicNotebook, EditableTreeview into master to add NumberedText, DynamicNotebook, EditableTreeview widget

PR Details: Widget name: NumberedText, DynamicNotebook, EditableTreeview Author: Faraaz Kurawle Description NumberedText: Text widget with the ability to display line numbers. DynamicNotebook: Notebook widget with the ability to add or remove tabs in the runtime. EditableTreeview: Treeview widget with the feature to edit a particular cell. Checklist [ x] Widget in a separate file in the appropriate folder [ x] Widget functions properly on both Windows and Linux [x ] Widget code includes docstrings with parameter descriptions [ x] Included an example file in /examples [x ] Widget is covered by unitttests in /tests [x ] Widget includes required assets files [ x] Reference to widget in AUTHORS.md [x ] Entry in sphinx documentation