Use full article url as baseUrl for Kirigami.SelectableLabel
Absoulute src urls (eg.https://example.org/some-url.png) and site
relative src urls (eg. /some-url.png) do work currently when using
only the sites baseUrl as base url.
However some sites do use page relative urls (eg. some-url.png) which
TextEdit will translate to https://example.org/some-url.png while it
should be https://example.org/article-path/some-url.png.
This patch makes it use the full page url instead of only the sites base
url as baseUrl for Kirigami.SelectableLabel ie. TextEdit.
This can be test with the feed from https://librearts.org/index.xml