Less canvas and viewers out

The release improves performance: instead of creating one canvas per slice/frame for the drawing, it just creates one. This was spotted by @rafasoyyo who also helped developing the solution, thanks!

This release also brings a big file/folder reorganisation to allow to use service workers. Since they need to be at the root of the folder they serve, viewers needed to adapt their structure. They now have independent repositories: dwv-simplistic, dwv-jqui and dwv-jqmobile. While I was at it, I now better use npm and have moved most of the external dependencies out to get them via a npm install or yarn. This simplifies dependency handling and makes more sense with the bot monitoring them (greenkeeper).

See milestone 0.23.0 for more details!