Toadlet: Add function showAsToadlet(ToadletContext) to replace the one without a ToadletContext parameter

This is useful for plugins which need to decide which Toadlet to return using the session cookies which can only be obtained through the ToadletContext.

We spoke on IRC about how this calls `showAsToadlet()` instead of just returning `this` to continue supporting existing code that overrides `showAsToadlet()`. I didn't realize this from reading it, so a comment would be helpful.