Pluralize Project API

This changes all APIs concerning paths and repositories on the project to be plural, preparing us to switch to multi-folder projects. It doesn’t make any changes to actually support multiple folders. Instead we just wrap the previous return values in singleton arrays.

* constructor `path` params -> `paths`
* `getRootDirectory` -> `getDirectories`
* `getPath` -> `getPaths`
* `setPath` -> `setPaths`
* `getRepo` -> `getRepositories`

It also adds the `Project::onDidChangePaths` event.