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.

theme/bootstrapbase/less/moodle/core.less

@@ -181,7 +181,7 @@ img.resize {
 /* Icon styles */
 img.icon {
     height: 16px;
-    vertical-align: text-bottom;
+    vertical-align: middle;
     width: 16px;
     padding-right: 6px;
 }
@@ -198,7 +198,7 @@ img.iconsmall {
 img.iconhelp, .helplink img {
     height: 16px;
     padding-left: 3px;
-    vertical-align: text-bottom;
+    vertical-align: middle;
     width: 16px;
 }
 .dir-rtl img.iconhelp, .dir-rtl .helplink img {