Requests on the Flickr resource provider are not cancellable

[Olivier Tilloy]

The Flickr resource provider in its current state does not implement the cancellation API that allows to have a reactive UI, resulting in weird bugs when browsing Flickr.

Steps to reproduce:

0) Clear your pictures cache in ~/elisa-0.5/pictures_cache
1) Launch Elisa
2) Go to Pictures > Internet > Flickr > Last 7 Days Interesting
3) Start a slideshow, and quickly jump to the next image a repeated number of times
4) Notice how the same images seem to be displayed in sequence, instead of the correct ones (and the tracebacks that are logged)

An example traceback is:

Traceback (most recent call last):
  File "/home/osomon/dev/pigment/pigment-python/pgm/timing/controller.py", line 278, in update
    modifier.update(factor)
  File "/home/osomon/dev/pigment/pigment-python/pgm/timing/modifier.py", line 206, in update
    obj.__setattr__(self._prop, value)
  File "/home/osomon/dev/elisa-branches/upicek_flickr_fixes/elisa-plugins/elisa/plugins/pigment/widgets/list.py", line 296, in visible_range_start__set
    len(self._widgets)-1)
  File "/home/osomon/dev/elisa-branches/upicek_flickr_fixes/elisa-plugins/elisa/plugins/pigment/widgets/list.py", line 275, in _render_widgets
    self._renderer(item, widget)
  File "/home/osomon/dev/elisa-branches/upicek_flickr_fixes/elisa-plugins/elisa/plugins/poblesec/player_slideshow.py", line 179, in _renderer
    cancel_deferred(widget, "dfr_full")
  File "/home/osomon/dev/elisa-branches/upicek_flickr_fixes/elisa-plugins/elisa/plugins/poblesec/player_slideshow.py", line 174, in cancel_deferred
    dfr.cancel()
AttributeError: Deferred instance has no attribute 'cancel'

[Olivier Tilloy (osomon)]

Actually this impacts all internet services that use the http_client plugin since requests are not cancellable.