Handle flatpak+https scheme

[meisenzahl]

Closes #169

[meisenzahl]

I use GLib.File.copy to download the FlatpakRef. This was the easiest way without adding new external dependencies.

[danirabbit]

Works as expected, just one request to make sure we throw an error dialog or I can't remember if Sideload has an error view. Something that the user can see the error

[meisenzahl]

In order to be able to display an error to a user in the ErrorView, I unfortunately had to move the logic for parsing the Flatpak file to the MainWindow. @danirabbit let me know what you think.

[danirabbit]

Hey nice. I didn't realize that so many of these other errors were silent as well. Thanks for fixing those! Seems like everything is working as expected. Good job!