Any .js and .css file placed directly in dropins folder is loaded into your pages (in alphabetical order, after system files and before the ones in your pages folder).
You may place custom Twig html templates in here and use them in any of your pages (e.g. by {% import 'foo.html' %} or {% extends 'bar.html' %}).
System templates can be overridden by creating a template with same name in here (e.g. base.html in dropins overrides pages/base/base.html).
The widget folder is being used by SmarthomeNG page generation plugin "visu-smartvisu". The plugin empties the folder and places plugin-specific widgets there. Widgets placed there manually will be deleted. If you are using the plugin "visu-smartvisu" you should place your own widgets in the dropins folder. There, you can add documentation files for these widgets. If no documentation is available / needed you can place the widgets also in "pages/(yourPage)/widgets".
Any valid .html file in widgets folder gets imported as widget library. E.g. a macro bar() defined in widgets/foo.html can be called in your pages by {{ foo.bar() }}.
Widget filenames have to be valid Twig/PHP variable names. They must not contain any non-alphanumeric characters (except underlines) and must not start with a number.
See the Wiki section on github for documentation on how to create your own widgets.
You may place your black icons in icons/sw/ and their white counterpart in icons/ws/.
Icons delivered by smartVISU get overridden if you use existing filenames. To make this work don't use icon0 nor any path in widgets, but just the filename.
To make SVG tintable by smartVISU, set fill and stroke by dedicated attributes and not by style (e.g. <path fill="#FFF" stroke="#FFF" instead of <path style="fill: #FFF; stroke: #FFF;").
Hint: Shrinking the files using SVGOMG with default settings this is replaced for you.
For extension of the existing language files you can place your own .ini file in the "lang" subfolder. smartVISU config page will find the file and show it in the language selector. See readme.txt in the smartVISU/lang folder.