Remove the deprecated usages of element_* methods

I've posted this patch to the #2109793: Convert element_* methods in common.inc to a class, but here is the better place for it. It replaces element_children in the common.inc only, but can be a good starting point for new patches.

Adding #2093143: [meta] Remove calls to @deprecated and "backwards compatibility" procedural functions from core as a parent issue.

#2094585: [policy, no patch] Core review bonus from https://drupal.org/comment/8466951#comment-8466951

I believe https://drupal.org/comment/8592475#comment-8592475 is duplicate of this issue? So uploading patch here as well

Using Function Replacer Part of Write script to automate replacement of deprecated functions where possible.

~/d8codetools/function_replace.php element_property 'Drupal\Core\Render\Element' RenderElement property
~/d8codetools/function_replace.php element_properties 'Drupal\Core\Render\Element' RenderElement properties
~/d8codetools/function_replace.php element_child 'Drupal\Core\Render\Element' RenderElement child
~/d8codetools/function_replace.php element_children 'Drupal\Core\Render\Element' RenderElement children
~/d8codetools/function_replace.php element_get_visible_children 'Drupal\Core\Render\Element' RenderElement getVisibleChildren
~/d8codetools/function_replace.php element_set_attributes 'Drupal\Core\Render\Element' RenderElement setAttributes

Yes, this is a duplicate, but didn't show up in a search because it had no issue summary. Let's use #2221755: Remove uses of deprecated Element functions