Fix `cordova prepare` now that plugin assets are copied on (un)install

This change introduces a new platform/foo/.staging/www directory, into
which plugin assets are installed by plugman. cordova prepare copies
these on top of the platform-specific www directory.

The platform-specific www directories are still deleted and recreated on
every prepare.

Uninstallation is working, plugman tests are passing, CLI tests are
broken but this change isn't the problem.
Access config.xml from serve. Serve from platform dir only