Create subdirectory for each widget.

Each widget subdirectory has:

- the template for that widget (if it has a template)
- a themes/ directory, which contains:
	- a subdirectory for each theme, containing a WidgetName.less file
	  and its compiled version, WidgetName_css.js
	- a WidgetName_template.less file based on the old themes/common/WidgetName.less,
	  included by WidgetName.less files

The old themes/*/variables.less files have been trimmed to just contain properties useful
across multiple widgets, like @dark-color or @holo-color.

Fixes #14.