Bring back dragging of elements from the clipboard to the activities - SL #4485

This is another API fix for code not yet ported to GTK+3, similar
to #3819:

- gtk.SelectionData.type  ->  Gtk.SelectionData.get_data_type()
- context.set_icon_pixbuf(...)  ->  Gtk.drag_set_icon_pixbuf(context, ...) [2]

[1] https://developer.gnome.org/gtk3/3.5/gtk3-Selections.html#gtk-selection-data-get-data-type
[2] https://developer.gnome.org/gtk3/stable/gtk3-Drag-and-Drop.html#gtk-drag-set-icon-pixbuf

Signed-off-by: Manuel Quiñones <manuq@laptop.org>