Allow context key expression that compares object properties

@jrieken I was attempting to change the Timeline's `timeline.excludeSources` setting from an array to an object (as requested [here](https://github.com/microsoft/vscode/commit/59c57e1e899db397deb331f13e1c8c431c7dcb93#commitcomment-37761862)). I've pushed a branch with the changes here: [eamodio/timeline-excludesources](https://github.com/microsoft/vscode/commits/eamodio/timeline-excludesources) Here I am trying to use the `timeline.excludeSources.<source-id>` property as the `toggled` condition. https://github.com/microsoft/vscode/blob/2d551abb57bfde80a48f2f5517b4c5fcab2380a4/src/vs/workbench/contrib/timeline/browser/timelinePane.ts#L1177 This seems to work for the initial state, but it never gets updated when the property is removed or re-added. It looks like the cause is somewhere in here: https://github.com/microsoft/vscode/blob/db36e74f62b1b2603906d4d2c892ab12cf632a12/src/vs/platform/contextkey/browser/contextKeyService.ts#L102-L120 Where `this._values` will contain `timeline.excludeSources.<source-id>`, but the `event.affectedKeys` will only have `timeline.excludeSources`. /cc @rebornix