Use GTK theme selection color for selected file-item and rubberband

Desktop-icon sets the color of a selected file-item and rubberband using its own stylesheet, however, the Gtk theme already sets this class styles, therefore it would be useful for the extension to reuse the same.

With this change, Desktop-icon takes the background color from 'rubberband' and 'view' GTK classes and uses them for 'rubberband' selection and 'file-item' selected respectively.

Note: While the 'view' GTK class it is not the closer class to file-item semantically, it is the closes one that we can get with 'selected' state.

closes #13 (closed)