Add BrowserWindow.showInactive()

This PR makes `BrowserWindow.show()` focusing the window and adds `BrowserWindow.showInactive()` that shows but not gives focus to the window.

I did this change because on some platforms showing and focusing a window is a bundled action, if we first shows the window without focusing it and then gives focus to it with another command, the window may not be focused finally.

This should be able to fix atom/atom#3616.