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>
Journal: fix the API for drag and drop - SL #3999

TestCase:

- open Paint activity, draw something
- go to the Journal
- drag the Paint entry to the Documents drive
- insert a USB stick
- drag the Paint entry to the USB drive

The paint should be in Documents and in the stick

Note: this needs the upstream fix
https://bugzilla.gnome.org/show_bug.cgi?id=692844 which is not in GTK+
3.6 but is added in current olpc builds as gtk3-3.6.4.1.fc18.olpc2.
So it can be tested on an XO.

Note: this does not fix the unwanted palette popup as described in the
ticket.  That will be a separate patch.

Signed-off-by: Manuel Quiñones <manuq@laptop.org>
More frame drag and drop fixes - SL #3819

- Errors introduced in recent commit 0aca8e59,
  jarabe.frame.clipboardobject.Format class doesn't have get_uri()
  method.

- jarabe.frame.clipboardobject.ClipboardObject: if format is
  'text/uri-list', Format.get_data() will contain only one URL.

Signed-off-by: Manuel Quiñones <manuq@laptop.org>
Merge pull request #21 from manuq/3819-3

More frame drag and drop fixes - SL #3819
Merge pull request #20 from manuq/3999

Journal: fix the API for drag and drop - SL #3999