Replace click-to-field parser swap with author-placed markers

[muzzwood]

This PR just concerns the experimental "Click to field" feature.

We're no longer swapping out the active parser. This means that some fields need to be manually marked in your template/markup.

Core fields and TVs are now marked in the template (breaking change). Without the parser swap, MagicPreview can't find core fields like the page title or TVs by itself. You now mark them with a new snippet, mpField, placed inside the element that shows the field:

<h1 [[mpField? &name=`pagetitle`]]>[[*pagetitle]]</h1>

With Fenom:

<h1 {'pagetitle' | mpField}>{$_modx->resource.pagetitle}</h1>

The snippet outputs nothing except inside the preview panel, so it's safe to leave in live templates. Sites that relied on core fields and TVs being clickable automatically will need to add it. The setting's description in the manager explains this.

ContentBlocks still works automatically for most fields. Each field is wrapped in an invisible element that makes it clickable. A field template can use the new [[+mpClickToFieldAttributes]] placeholder to put the click target on its own element instead of getting the extra wrapper:

<h2 [[+mpClickToFieldAttributes]]>[[+value]]</h2>

• Where it works: templates typed into the field's settings, and @FILE and @PDO_FILE template files, including Fenom ones.

• @PDO_FILE fields: @PDO_FILE templates, the usual way to write ContentBlocks templates in Fenom, aren't wrapped automatically. Add the placeholder to make them clickable:

  <{$level} [[+mpClickToFieldAttributes]] class="headline">{$value}</{$level}>
  

Only the preview panel has click targets. Previews in a new window, and a draft's "View" link, now show a clean page. There's no editor beside them to scroll to anyway.