Add experimental support for RTL languages

This commit permits language plugins to carry the field
“text-direction” with the value “rtl” to trigger right-to-left layout
of the entire page. We also adjust the sidebar layout to work in RTL
mode.

There are still a number of problems to be addressed:

* Brackets and other punctuation incorrectly placed within en-GB UI text
* System tiddler titles are rendered semi-back-to-front (eg
`languages/ca-ES/:$`)

Starting to address #1845 and the discussion in #2523.

core/modules/startup/startup.js

@@ -63,7 +63,18 @@ exports.startup = function() {
 		controllerTitle: "$:/language",
 		defaultPlugins: [
 			"$:/languages/en-US"
-		]
+		],
+		onSwitch: function(plugins) {
+			if($tw.browser) {
+				console.log("switching language",plugins)
+				var pluginTiddler = $tw.wiki.getTiddler(plugins[0]);
+				if(pluginTiddler) {
+					document.documentElement.setAttribute("dir",pluginTiddler.getFieldString("text-direction") || "auto");
+				} else {
+					document.documentElement.removeAttribute("dir");
+				}
+			}
+		}
 	});
 	// Kick off the theme manager
 	$tw.themeManager = new $tw.PluginSwitcher({