Update after internal discussion

It's actually a bad idea to move old configuration files to an old place. Because after an update, plugins will fallback to default plugin configuration instead of keeping the right configuration in place.

So, let's do the opposite:

* 1st step: during an install, if we detect already existing `config/pluginname` directory, we copy plugin ZIP file content in `config/pluginname.new`.
* 2nd step: could be within the same PR or in a new one, we do the same thing but file by file; it means that we copy all non existing files from the plugin to `config/pluginname` but each time a file already exists, we simply copy the new file to the same dir but we append a `.new` at the end.

This patch implements the 1st step.