Reimplement ImageLoader

[janezd]

Add-on orange3-pumice now usesorangecontrib.imageanalytics.widgets.owimageviewer.image_loader and ImageLoader for loading images (biolab/orange3-pumice#6). What about moving them and related functions to core Orange?

They may be useful in other add-ons, too. E.g., the Geo add-on includes an ImageLoader with a comment

# Mostly a copy from OWImageViewer in imageanalytics add-on

[janezd]

If we do so, here's a bug to fix.

        qnam = ImageLoader.networkAccessManagerInstance()
        future, deferred = image_loader(QUrl(url), qnam)
        f = deferred()
        w = FutureWatcher(f, )
        w.done.connect(self.__on_future_done)

The signal is never triggered. One has to either set a parent, qnam.setParent(self), (bad: this instance is shared!) or set up another manager, e.g. QNetworkAccessManager(self).

Apparently, the parentless network access manager dies a premature death. Or I am misusing it.

Perhaps a network access manager should be created for the entire app?

[ales-erjavec]

Please no. ImageLoader is bad and should not be used elsewhere or be advertised (i.e. moved to common util package)