Provide public API for accessing _props, _classes and _style.

[falkoschindler]

This PR introduces a Props class derived from dict to replace the props() method. By implementing the __call__ method, the new .props property can be called like before, but also accessed like a dictionary. This allows reading and writing the props dictionary without accessing the private _props attribute. Furthermore it moves most of the rather complex parsing logic into a separate file, relieving the pretty long element.py module.

Currently this PR is only a draft. If we like the idea, we should

• fix pytests
• apply the same concept to .classes and .style
• fix linting errors in ui.query
• fix linting errors in ui.tree and ui.menu
• use the new API in tests, documentation and examples