Poshuku backends split

[Georg Rudoy]

Hi group,

In order to support QWebEngine [1] in Poshuku I've split the actual HTML rendering backend out of Poshuku core to a separate plugin: meet Poshuku WebKitView. It is enabled by default and should be enabled basically everywhere where QtWebKit is available.

This leads to a couple of interesting consequences:

1. Obvious one: QWebEngine support will be added somewhat soon, bringing Chromium-like standards compatibility, stability, responsiveness, and memory consumption too (well, QWebEngine is quite close to Chromium in a sense).

QWebEngine is quite on par with QtWebKit in Qt 5.7+ feature-wise, so everything is expected to be working, including proper AdBlock and Greasemonkey userscripts support.

2. Less obvious one: I've tried to make the HTML rendering backends API as Qt-agnostic and WebKit-agnostic as possible, so in theory it's now possible to embed any HTML rendering backend out there as long as it's wrappable in a QWidget. Gecko, anyone?

Of course, at least one backend plugin should be available at all times, otherwise LC crashes and would probably crash more gracefully in the future, but this configuration isn't supported and shouldn't be supported anyway.

[1] http://doc.qt.io/qt-5/qtwebengine-overview.html

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  Georg Rudoy