downloader: Make download cancellable

https://bugzilla.gnome.org/show_bug.cgi?id=729026
util: Generalize cache getting

https://bugzilla.gnome.org/show_bug.cgi?id=729026
wizard-toolbar: Only click buttons when appropriate

Buttons should only be clicked if they are sensitive.

https://bugzilla.gnome.org/show_bug.cgi?id=729026
wizard-source: Introduce download_required prop

This function will be used by the following patches to determine
whether a remote source has to be downloaded or not. A source is
considered to require download if it uses the HTTP(S) URI scheme.

https://bugzilla.gnome.org/show_bug.cgi?id=729026
downloader: Add fetch_media()

This allows easy downloading of media to the users Download folder. If
the user already downloaded the media (via Boxes or externally) and left
it with the same name as the initial one in the Download folder it will
be taken from there.

https://bugzilla.gnome.org/show_bug.cgi?id=729026
wizard: prepare() now reports progress

Previously, only prepare_media reported progress. This patch adds
progress reporting to the whole 'prepare' call chain.

This patch makes it possible to introduce a subprogress for media
downloading in a later patch.

https://bugzilla.gnome.org/show_bug.cgi?id=729026
wizard: Codestyle changes

- Reduce indentation for better readability
- Correct indentation error
- Move properties up
- Remove unneeded this.

https://bugzilla.gnome.org/show_bug.cgi?id=729026
wizard: Allow entering of HTTP(S) URIs

And also validate URI has a proper basename.

https://bugzilla.gnome.org/show_bug.cgi?id=729026
wizard: Introduce prepare_cancellable

This cancellable will be used for media downloading in the following
patches. It may also be used later for cancelling other preparation
actions.

https://bugzilla.gnome.org/show_bug.cgi?id=729026
wizard: Introduce download_media ()

This adds a function to download media via HTTP or HTTPS protocol from
the internet. Functions of other objects used here were gradually
introduced with the parent commits of this.

https://bugzilla.gnome.org/show_bug.cgi?id=729026
wizard: Introduce media downloading feature

This adds the possibility to download installation/live media via the
HTTP(S) protocol from the internet.

https://bugzilla.gnome.org/show_bug.cgi?id=729026