Allow changing download network preference

[taitsmith]

Changes
Adds a section in the settings screen which gives users the option of changing their preferred download network. Previously, users were unable to allow or restrict mobile data downloads after making the initial choice. Additionally updates the message displayed when the initial dialog is displayed to inform users their choice can be updated.

Issues
Fixes #1326

[taitsmith]

The issue I ran into is that singleChoice() expects a string key, and DownloadMethod is stored as an Int. As it is, the PR creates items with string keys and separately handles the updating of the download preference in defaultOnSelectionChange, which obviously is not ideal given that some amount of unnecessary work is being done and unneeded preferences are being created. Potential other solutions were updating the library to create something like singleChoiceInt() which seems like quite a bit of work for this one issue, or providing multiple check boxes like the player options currently provide.

[Maxr1998]

The issue I ran into is that singleChoice() expects a string key, and DownloadMethod is stored as an Int.

True, that's definitely a shortcoming of the library. Not a huge problem though and we can change this later when the library gets updated to directly support ints, I believe.. the only thing that is a bit of a pain is that this new preference will store the setting twice (as int in pref_download_method and as string in download_method, the latter being unused), and currently doesn't restore the current preference value from the int preference. Could you look into fixing the second point? Should be relatively simple.

Otherwise, this looks pretty good.

Edit: the new 2.4.0-beta2 of the preferences libraries will support singleChoice with int keys, so we can also use that. I'll update to that version once the distribution is complete, and you can rebase on top of the latest master branch and make the required changes.

Edit 2: done, you can now proceed.

[taitsmith]

awesome thanks, was actually going to sit down today and look into updating the library so i'll get this wrapped up

[Maxr1998]

Rebased your PR onto the latest master branch and fixed the conflicts. Thanks for your contribution!