[TIMOB-18062] iOS: AttributedString Parity with Android

Jira: https://jira.appcelerator.org/browse/TIMOB-18062

This PR is a continuation of https://github.com/appcelerator/titanium_mobile/pull/6409/ which came from #6358.

It addresses the comments by @pingwang2011 for the Android portion of the code. The doc is still not completed yet as Titanium.UI.iOS.Attribute and Titanium.UI.iOS.AttributedString yml files are missing the "deprecated".

The propertyAccessors is now used in AttributedStringProxy.java, AttributeProxy.java and LabelProxy.java.
If the user set "attributedString" and "text" initially, then "attributedString" is respected while "text" is ignored. But if the user later change the property "text", "text" will be respected and "attributedString" will be ignored based on the current implementation of propertyChanged. Is that expected? How about the "font" and "backgroundColor"?