MDL-41438 core_renderer: Update the parameters for heading() and heading_with_help().
heading(), change the default value for classes from 'main' to null.
heading_with_help(), add 2 additional parameters for level and classnames. Default value of level is set to 2 and classnames is set to null.

With the changes for heading_with_help(), icon image vertical alignment needs to be adjusted, so it display in the middle of the heading.

Also added some notes in theme/upgrade.txt file.