Introduce new manualdownload saver

This saver pops up a modal dialogue giving the user an opportunity to
right click and save the wiki

core/messages/Download.tid

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+title: $:/messages/Download
+type: text/x-tiddlywiki
+subtitle: Download changes
+footer: <<button close class:"btn btn-primary"><Close>>
+help: http://five.tiddlywiki.com/#help:DownloadingChanges
+
+Your browser only supports manual saving.
+
+To save your modified wiki, right click on the download link below and select "Download file" or "Save file", and then choose the folder and filename.
+
+//You can marginally speed things up by clicking the link with the control key (Windows) or the options/alt key (Mac OS X). You will not be prompted for the folder or filename, but your browser is likely to give it an unrecognisable name -- you may need to rename the file to include an `.html` extension before you can do anything useful with it.//
+
+On smartphones that do not allow files to be downloaded you can instead bookmark the link, and then sync your bookmarks to a desktop computer from where the wiki can be saved normally.

core/modules/savers/manualdownload.js

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+/*\
+title: $:/core/modules/savers/manualdownload.js
+type: application/javascript
+module-type: saver
+
+Handles saving changes via HTML5's download APIs
+
+\*/
+(function(){
+
+/*jslint node: true, browser: true */
+/*global $tw: false */
+"use strict";
+
+var downloadInstructionsTitle = "$:/messages/Download"
+
+/*
+Select the appropriate saver module and set it up
+*/
+var ManualDownloadSaver = function(wiki) {
+};
+
+ManualDownloadSaver.prototype.save = function(text) {
+	$tw.modal.display(downloadInstructionsTitle,{
+		downloadLink: "data:text/html," + encodeURIComponent(text)
+	});
+	return true;
+};
+
+/*
+Information about this saver
+*/
+ManualDownloadSaver.prototype.info = {
+	name: "manualdownload",
+	priority: 0
+};
+
+/*
+Static method that returns true if this saver is capable of working
+*/
+exports.canSave = function(wiki) {
+	return true;
+};
+
+/*
+Create an instance of this saver
+*/
+exports.create = function(wiki) {
+	return new ManualDownloadSaver(wiki);
+};
+
+})();