dconf-editor: Use margin-end instead of margin-right

The key_info_grid grid uses the deprecated "margin-right" property. It
should use "margin-end" which properly supports both left-to-right and
right-to-left text directions.

https://bugzilla.gnome.org/show_bug.cgi?id=739421
dconf-editor: Don't use stock image

Stock images are deprecated. Dconf-editor uses the GtkImage:stock
property in the close button in the search box. Replace it with the
GtkImage:icon-name property instead.

https://bugzilla.gnome.org/show_bug.cgi?id=739422