Preparing for 5.1.6
Fix problem with edit-bitmap widget static rendering

The ‘introduction’ edition contains an edit-bitmap widget that was
failing under Node.js because the fake dom doesn’t support canvas.
Add Dutch TiddlyWiki blog post
Adjust default dropdown width

To avoid text of exporter descriptions overflowing on Firefox.

We need a better solution. On Chrome, nested popups resize to fit their
content, without being constrained by the parent width.