Add a cascade for rendering fields within the Edit Template (#6511)

* Allow the rendering of fields to be extended

This commit extends the `$:/core/ui/EditTemplate/fields` tiddler to
use a new cascade (Field Editor Cascade) to allow customizing the
rendering of the field editor.
It provides a default element for the cascade that displays the standard
EditTextWidget as a fallback. That way, the implementation is completely
backwards compatible. The `currentTiddler` and `currentField` variables
are available in the transcluded tiddler. This has the additional
benefit, that not only the `EditTextWidget` can be used. The user can
use a dropdown-list or even something completely crazy. As long as it
can be put into a tiddler that updates the field, it will be fine.

* Make `select` Tags in Fields look like the rest

This patch updates the CSS to make `tc-edit-texteditor` usable on
`select`-tags as well.

I'm not sure what `-webkit-appearance: none;` is for, but it hides the
DropDown-arrow in Chrome and makes the select-tag hard to discover. I've
changed the css to only apply it to the input tag. Maybe it can be
removed altogether.

* Add documentation for the Field Editor Cascade

core/ui/ControlPanel/Cascades/FieldEditor.tid

@@ -0,0 +1,9 @@
+title: $:/core/ui/ControlPanel/FieldEditor
+tags: $:/tags/ControlPanel/Cascades
+caption: {{$:/language/ControlPanel/FieldEditor/Caption}}
+
+\define lingo-base() $:/language/ControlPanel/FieldEditor/
+
+<<lingo Hint>>
+
+{{$:/tags/FieldEditorFilter||$:/snippets/ListTaggedCascade}}