Keyboard shortcut for starring files.

Currently, there is no keyboard shortcut for starring/adding files as favorites.

However, for a lot of users who prefer using the keyboard instead of a mouse, using the popover/context menu is more cumbersome.

To fix this, a new keyboard shortcut shift+control+s is added for starring files.
This has also been updated on the shortcuts help window.

GNOME/nautilus#2194