Show complete article

Many feeds do not contain the complete article text but only a teaser (e.g. https://www.heise.de/rss/heise.rdf). It is useful to fetch the complete article text such that you don't have to open the feed in the browser.

This MR contains:

• Infrastructure which allows to easily exchange readability implementations via strategy pattern.
• GUI update if content of an entry changed.
• Reference implementation for qreadable which fetches the complete article in the background without blocking the GUI. It uses the official Mozilla readability which is the most actively maintained and well tested readability implementation.

Open topics (to be handled in future MRs):

• turn qreadable into an official KDE project and use the library instead of the executable
• show indication if content is being fetched; should probably be a global indication and not per entry
• fetch content if it's not available (e.g. if Alligator has been closed before content for all entries could be fetched); solution: add flag in DB

see also !22 (closed)