Plugin API: Add support for onFocusGained and onFocusLost event

[bryphe]

From PR #1106 - it'd be nice to have a way to get focus gained / focus lost events for plugins.

Perhaps something on the Workspace API, like:

oni.workspace.onFocusGained.subscribe(() => { ... })
oni.workspace.onFocusLost.subscribe(() => { ... })

These would basically be proxies to the browser window focus events blur and focus: https://github.com/electron/electron/blob/master/docs/api/browser-window.md#event-blur

As part of this, we should update the git statusbar plugin to listen to these events to update the branch state.