Add flash plugin support

[PoisonousJohn]

Flash is not working now

[timsueberkrueb]

QtWebEngine supports Flash.
See http://doc.qt.io/qt-5/qtwebengine-features.html#pepper-flash-player-plugin-support.
Haven't tested it yet, though.

[plfiorini]

Flash is really bad regarding security and stability, unfortunately some sites are still using it, the most notable being Facebook. Of course we should support Flash but it should be disabled by default.

[PoisonousJohn]

I tried to add

Qt += webenginewidgets

and

QWebEngineSettings::defaultSettings()->setAttribute(QWebEngineSettings::PluginsEnabled, true)

and it's not working for me when I'm checking it here https://www.adobe.com/software/flash/about/

[timsueberkrueb]

@JohnPoison cool. We will need to handle it in a Oxide-compatible way (e.g. Qt += webenginewidgets only if QTWEBENGINE_ENABLED, same with the attribute).
I agree with @plfiorini it should be disabled by default.

[PoisonousJohn]

Sure, it was just for test. I also agree that it should be disabled by default . But the problem is that it's not working and I can't understand why. Flash in DemoBrowser from Qt examples works on adobe test page. Yet it doesn't work on http://rutube.ru