function ExtensionManagerViewModel() {
        this._handleStatusChange = this._handleStatusChange.bind(this);

        // Listen for extension status changes.
        ExtensionManager
            .on("statusChange." + this.source, this._handleStatusChange)
            .on("registryUpdate." + this.source, this._handleStatusChange);
    }
The base model for the ExtensionManagerView. Keeps track of the extensions that are currently visible
and manages sorting/filtering them. Must be disposed with dispose() when done.
Events:
- change - triggered when the data for a given extension changes. Second parameter is the extension id.
- filter - triggered whenever the filtered set changes (including on initialize).

@constructor