🍎 Add 'Hide' to App API for OS X only

[thingsinjars]

When creating a popover window via global hotkey, you might want to return focus to the previous app after closing all windows.

For example:
http://stackoverflow.com/questions/22081215/return-focus-to-previous-apps-window-after-makekeyandorderfront

Without app.hide(), you need to interact (click on the window or cmd-tab away and back) to refocus.

With app.hide(), you can return focus directly (gif says "hide before quit", should be "hide before close"):

I don't think there's an equivalent for Linux or Windows so I've only done this for OS X and tagged it as such in the docs.

[thingsinjars]

I had a look at the solution for hiding mentioned in #3694 but that doesn't work in the case where the app is a menubar-only app without a menu. In this case, even pressing cmd-H doesn't hide the app so it really needs an api call for it.

[thingsinjars]

I also added in the call for app.show(). It doesn't automatically focus the app window again as it simply passes through to [NSApp unhide:nil];. It's always possible to just call app.focus() afterwards, anyway.

[zcbenz]

👍