Allow manual entry of proxy server.

[astaric]

On networks, where proxy use is mandatory, Orange cannot access the network.

We could add a setting to canvas where users could specify which proxy server to use and modify the widgets to access this setting and use proxy, if the setting has a value.

Addon dialog, file widget, image embedings (addon), twitter (addon) and some more need to be modified.

[kernc]

If we have the setting set http_proxy environment variable and require a restart, this is all that's needed for requests / urllib code to use it.

[astaric]

Set it when canvas is opened or did you have a more permanent way in mind? Why is the restart necessary?

[kernc]

Yes, when the canvas is open, check the setting and set the environ. The restart is necessary because any existing Request/Session objects wouldmight not honor this setting.

[astaric]

Fixed in #2906