API Using $HolderID for form field container templates

Replaces duplicate $ID usage.
See https://github.com/silverstripe/silverstripe-framework/pull/1873

templates/forms/CheckboxField_holder.ss

@@ -1,4 +1,4 @@
-<div id="$Name" class="field<% if extraClass %> $extraClass<% end_if %>">
+<div id="$HolderID" class="field<% if extraClass %> $extraClass<% end_if %>">
 	$Field
 	<label class="right" for="$ID">$Title</label>
 	<% if Message %><span class="message $MessageType">$Message</span><% end_if %>

templates/forms/CheckboxSetField.ss

@@ -1,4 +1,4 @@
-<ul id="$ID" class="$extraClass">
+<ul id="$HolderID" class="$extraClass">
 	<% if Options.Count %>
 		<% loop Options %>
 			<li class="$Class">

templates/forms/OptionsetField.ss

@@ -1,4 +1,4 @@
-<ul id="$ID" class="$extraClass">
+<ul id="$HolderID" class="$extraClass">
 	<% loop Options %>
 		<li class="$Class">
 			<input id="$ID" class="radio" name="$Name" type="radio" value="$Value"<% if isChecked %> checked<% end_if %><% if isDisabled %> disabled<% end_if %> />

templates/forms/TreeDropdownField.ss

@@ -1,4 +1,4 @@
-<div id="TreeDropdownField_$ID"
+<div id="$HolderID"
      class="TreeDropdownField single<% if extraClass %> $extraClass<% end_if %><% if ShowSearch %> searchable<% end_if %>"
      data-url-tree="$Link(tree)"
      data-title="$TitleURLEncoded"