Add shortcut key for saving URL.

This adds the shortcut key for saving a URL in wallabag.  The key is currently set to Accel-Alt-Shift (where accel is a platform-specific shortcut key, ex. Ctrl on Windows/Linux).  It was chosen mostly to avoid conflicts with other keys.

I also cleaned up the mixed tab/space formatting in overlay.xul to be all spaces, which makes it much easier to read across editors.