Introduce new subfilter operator (#3508)

core/modules/filters/subfilter.js

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+/*\
+title: $:/core/modules/filters/subfilter.js
+type: application/javascript
+module-type: filteroperator
+
+Filter operator returning its operand evaluated as a filter
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+/*
+Export our filter function
+*/
+exports.subfilter = function(source,operator,options) {
+	var list = options.wiki.filterTiddlers(operator.operand,options.widget,source);
+	if(operator.prefix === "!") {
+		var results = [];
+		source(function(tiddler,title) {
+			if(list.indexOf(title) === -1) {
+				results.push(title);
+			}
+		});
+		return results;
+	} else {
+		return list;
+	}
+};
+
+})();

editions/tw5.com/tiddlers/filters/examples/subfilter Operator (Examples).tid

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+created: 20181031175129475
+modified: 20181031175129475
+tags: [[subfilter Operator]] [[Operator Examples]]
+title: subfilter Operator (Examples)
+type: text/vnd.tiddlywiki
+
+<<.operator-example 1 "[subfilter[one two three]addsuffix[!]]">>
+<<.operator-example 2 "[subfilter{$:/StoryList!!list}]">>

editions/tw5.com/tiddlers/filters/subfilter Operator.tid

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+caption: subfilter
+created: 20181031175129475
+modified: 20181031175129475
+op-input: a [[selection of titles|Title Selection]] passed as input to the subfilter
+op-neg-input: a [[selection of titles|Title Selection]] passed as input to the subfilter
+op-neg-output: those input titles that are <<.em not>> returned from the subfilter  <<.place S>>
+op-output: the [[selection of titles|Title Selection]] returned from the subfilter <<.place S>>
+op-parameter: a [[filter expression|Filter Expression]]
+op-parameter-name: S
+op-purpose: select titles from the operand interpreted as a [[filter expression|Filter Expression]]
+tags: [[Filter Operators]] [[Field Operators]] [[Selection Constructors]] [[Negatable Operators]]
+title: subfilter Operator
+type: text/vnd.tiddlywiki
+
+<<.from-version "5.1.18">> Note that the subfilter operator was introduced in version 5.1.18 and is not available in earlier versions.
+
+<<.tip " Literal filter operands cannot contain square brackets but you can work around the issue by using a variable:">>
+
+```
+<$set name="myFilter" value="[tag[one]]">
+<$list filter="[all[tiddlers]subfilter<myFilter>]">
+...
+```
+
+<<.operator-examples "subfilter">>