akregator: port away from ReadOnlyPart.
Summary: This removes the konqueror integration which didn't really work anyway (RMB on .rss file / Preview In / Akregator -> akregrator would be embedded but wouldn't add this feed). RMB / Add to Akregator is simpler and works better anyway. This will allow us to load kontact plugins without desktop files (using json) in the future. Depends on D28581. Test Plan: akregator still works (standalone and in kontact) and I was able to add a new feed in the GUI (and the icon worked btw, also after a restart). Reviewers: mlaurent Differential Revision: https://phabricator.kde.org/D28584
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