Add new "variables" and "getvariables" operators

core/modules/filters/getvariable.js

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+/*\
+title: $:/core/modules/filters/getvariable.js
+type: application/javascript
+module-type: filteroperator
+
+Filter operator for replacing input values by the value of the variable with the same name, or blank if the variable is missing
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+/*
+Export our filter function
+*/
+exports.getvariable = function(source,operator,options) {
+	var results = [];
+	source(function(tiddler,title) {
+		results.push(options.widget.getVariable(title) || "");
+	});
+	return results;
+};
+
+})();

core/modules/filters/variables.js

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+/*\
+title: $:/core/modules/filters/variables.js
+type: application/javascript
+module-type: filteroperator
+
+Filter operator for returning the names of the active variables
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+/*
+Export our filter function
+*/
+exports.variables = function(source,operator,options) {
+	var names = [];
+	for(var variable in options.widget.variables) {
+		names.push(variable);
+	}
+	return names.sort();
+};
+
+})();

editions/tw5.com/tiddlers/filters/examples/getvariable.tid

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+created: 20190330100101453
+modified: 20190330100101453
+tags: [[getvariable Operator]] [[Operator Examples]]
+title: getvariable Operator (Examples)
+type: text/vnd.tiddlywiki
+
+<<.operator-example 1 "[[currentTiddler]getvariable[]]" "returns the value of the variable ''currentTiddler''">>

editions/tw5.com/tiddlers/filters/examples/variables.tid

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+created: 20190330100101453
+modified: 20190330100101453
+tags: [[variables Operator]] [[Operator Examples]]
+title: variables Operator (Examples)
+type: text/vnd.tiddlywiki
+
+<<.operator-example 1 "[variables[]prefix[colour]]" "returns the names of any variables whose names start with ''colour''">>

editions/tw5.com/tiddlers/filters/getvariable.tid

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+created: 20190330100101453
+modified: 20190330100101453
+tags: [[Filter Operators]] [[Special Operators]]
+title: getvariable Operator
+caption: getvariable
+op-purpose: select all values of variables named in the input titles
+op-input: a selection of varible names
+op-parameter: ignored
+op-output: the values of each of the variables named in the input titles (or blank if the variable is not defined)
+
+The usual way to retrieve a variable value within a filter is with the angle brackets notation. For example, `[<currentTiddler>]` will retrieve the value of the variable called `currentTiddler`. 
+
+The `getvariable` filter operator provides an alternative way to retrieve a variable. For example, `[[currentTiddler]getvariable[]]` is another way to retrieve the value of the variable `currentTiddler`.
+
+The advantage of `getvariable` is that it makes it possible to work with variables whose name is computed, and not known in advance. For example, `[<myvariable>getvariable[]]` gets the value of the variable whose name is given in the variable `myvariable`.
+
+<<.operator-examples "getvariable">>

editions/tw5.com/tiddlers/filters/variables.tid

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+created: 20190330100101453
+modified: 20190330100101453
+tags: [[Filter Operators]] [[Special Operators]] [[Selection Constructors]]
+title: variables Operator
+type: text/vnd.tiddlywiki
+caption: variables
+op-purpose: select the names of all the actively defined variables
+op-input: ignored
+op-parameter: none
+op-output: the names of all the actively defined variables
+
+The primary purpose of the `variables` operator is to implement the [[dumpvariables Macro]]:
+
+<$codeblock code={{$:/core/macros/dumpvariables}}/>
+
+<<.operator-examples "variables">>