function InstalledViewModel() {
        ExtensionManagerViewModel.call(this);

        // when registry is downloaded, sort extensions again - those with updates will be before others
        var self = this;
        ExtensionManager.on("registryDownload." + this.source, function () {
            self._sortFullSet();
            self._setInitialFilter();
        });
    }
The model for the ExtensionManagerView that is responsible for handling previously-installed extensions.
This extends ExtensionManagerViewModel.
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