This updates the .npmignore file to ignore some new files that were
recently added, and to keep the `core` directory, so that noVNC can be
used as an ES6 module from NPM.
This also updates package.json to clean when generating lib files in the
`prepare` phase (which has replaced the `prepublish` phase).
This commit restructures noVNC, splitting it into the core directory
and the app directory, with the former containing core noVNC parts,
and the latter containing parts specific to the application.
Solly Ross
Pierre Ossman
Samuel Mannehed