[Merge] ~nteodosio/software-properties:attach-url into software-properties:ubuntu/master

[Nathan Teodosio]

The proposal to merge ~nteodosio/software-properties:attach-url into software-properties:ubuntu/master has been updated.

Description changed to:

Just setting markup on the child label of the GtkRadioButton is not enough;
The URI would nonetheless be not clickable because the radio button activation
action takes precedence.

The change can confuse users of screen readers, because it removes the child label of the radio buttons and instead separates them as independent widgets. As such, it also includes accessibility directives to fix that.

Alternative to https://code.launchpad.net/~xnox/software-properties/+git/software-properties/+merge/443188.

To test without building and installing package: Clone the repository, cd into it, run ./software-properties-gtk --data-dir=$PWD/data, make sure screen reader is on[1].

[1] https://help.ubuntu.com/stable/ubuntu-help/a11y-screen-reader.html.en

For more details, see:
https://code.launchpad.net/~nteodosio/software-properties/+git/software-properties/+merge/454886
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