Simplify in-place editor selection (Was: Refactor PlainTextEditor/WysiwygEditor/Editor in-place editors now that text fields no longer have configurable filteredness)

Blocked on #2313757: Remove text_processing option from text fields, expose existing string field types as plain text in UI.

See #37.5 and #48.5 regarding that.

+++ b/core/modules/quickedit/src/Plugin/InPlaceEditor/PlainTextEditor.php
@@ -32,8 +29,8 @@ public function isCompatible(FieldItemListInterface $items) {
-    // This editor is incompatible with processed ("rich") text fields.
-    elseif ($field_definition->getSetting('text_processing')) {
+    // This editor is incompatible with filtered ("rich") text fields.
+    elseif (in_array($field_definition->getType(), array('text', 'text_long', 'text_with_summary'))) {

This was only necessary because text fields could previously be both filtered and unfiltered (plain), depending on the configuration. Since this is no longer possible, this if-statement can be removed and we just don't tag the processed text fields with the plain_text in-place editor anymore.

Except that removing that no longer associating the plain_text in-place editor anymore with filtered text fields no longer allows the editor.module's Editor (WYSIWYG) in-place editor to automatically declare itself as an alternative to the plain text in-place editor.

In other words: we'll need to revise how all that works.