First pass at language plugins for internationalisation

Still quite a few details to work out, but this shows the basic idea of
re-using the theme mechanism to handle language plugins.

Comments and questions welcome.

core/wiki/languageswitcher.tid

@@ -0,0 +1,8 @@
+title: $:/snippets/languageswitcher
+
+{{$:/lingo/ControlPanel/Language/prompt}} <$view tiddler={{$:/language}} field="description" format="text"><$view tiddler={{$:/language}} field="name" format="text"><$view tiddler={{$:/language}} field="title" format="text"/></$view></$view>
+
+<$linkcatcher to="$:/language">
+<$list filter="[plugin-type[language]sort[title]]"><div><$reveal state="$:/language" type="match" text={{!!title}}>&bull;</$reveal><$reveal state="$:/language" type="nomatch" text={{!!title}}>&nbsp;</$reveal> <$link to={{!!title}}><$view field="description" format="text"><$view field="name" format="text"><$view field="title" format="text"/></$view></$view></$link></div>
+</$list>
+</$linkcatcher>