Add methods for handling deprecations

Plan:

1. borrow "_deprecated_function" and "_deprecated_argument" functions from WordPress
   • rename them to "kUtil::deprecatedMethod" and "kUtil::deprecatedArgument"
   • change used naming to use "method" instead of "function"
   • trigger silenced user deprecation notice instead of user notice
   • don't do anything if not in Debug Mode
2. locate code elements, that should be deprecated and for each of them:
   • indicate deprecation version in "@deprecated" tag of DocBlock (add one if missing)
   • add "@see ClassName::methodName()" tag to indicate replacement method to be called
   • instead of manually triggering a deprecation notice use methods, added above