FormWidget: add note that this.focusNode must be a sub-node rather than the root node.
Container: remove getPreviousSibling() and getNextSibling().
Can just use Element#previousElementSibling and #nextElementSibling.
fix tabbing in less files, fixes #196
Add d-hidden and d-invisible classes to global.less,
plus d-hidden, d-shown, d-visible, and d-invisible
attribute selectors to be used in templates.

Thought about putting the attribute selectors in a CSS file automatically
loaded by handlebars.js, but just putting theme as part of the theme for now.

Fixes #197.