multiple improvements

[edulix]

This pull includes several improvements:

• I've merged allowing the "orphan" resources to de rendered based on their position #13 pull request to support orphan resources (thanks for that @Mikayl, nice contribution!)
• Mouseover shows only tooltip when sidebar is collapsed (this is done by setting the sidebarIsOpen property, as in default ReactAdmin Menu, see https://github.com/marmelab/react-admin/blob/master/packages/ra-ui-materialui/src/layout/Menu.tsx#L91)
• Ensures that on first rendering, the parent menu item of the active menu item is open.
• shows logout only when isXSmall (as in react-admin Menu, see https://github.com/marmelab/react-admin/blob/master/packages/ra-ui-materialui/src/layout/Menu.tsx#L94)
• use theming for the TreeMenu component, and adds a bunch of theme classes to allow for theme overriding (which I do use in my code).
• Removes the commented lines related to the divider. (If the divider is useful, it could be added later as an optional property in CustomMenuItem).
• Based on current react-admin practices, I've transformed the code to be more functional by removing code making it a HOC.

[harshit-budhraja]

Hi @edulix !! Thank you so much for such rich contributions to improve the overall codebase for ra-treemenu. Really appreciate it! 💯
P.S. We've accommodated these changes in the upcoming release (1.0.3).