Switch from Ubuntu One dialog in plugins and use standard dialog

[Robert Ancell]

[Impact]
In GNOME Software 3.20 (Xenial) we have an Ubuntu One dialog that runs inside the plugins. GNOME 3.22 implemented a authentication dialog into the GNOME Software code in 3.22 (based on our dialog) which we switched to using. The dialog support was backported into 3.20.5.

When the current dialog is run it runs from a different thread to the main UI. GTK+ is not thread safe and so this is likely to cause random crashes. May be the cause of bugs like bug 1614130.

[Test Case 1]
1. Delete any existing Ubuntu One credentials by running seahorse and deleting keys named com.ubuntu.UbuntuOne.*
2. Open GNOME Software
3. Remove a snap

Expected result:
- The Ubuntu One dialog shows, works and the snap is removed

[Test Case 2]
1. Delete any existing Ubuntu One credentials by running seahorse and deleting keys named com.ubuntu.UbuntuOne.*
2. Open GNOME Software
3. Write an app review

Expected result:
- The Ubuntu One dialog shows, works and the review is written.

NOTE: Pick an app with few reviews so you can see your one that is added. May need to restart GNOME Software to see review (this is existing behaviour).

[Regression Potential]
Change is non-trivial, though the dialog code has been used in GNOME Software 3.22 and onwards. Some risk of breaking related code.

[Robert Ancell (robert-ancell)]

I have updated the patches to do this in the wip/rancell/snap-auth-3-20-rebase git branch.