Refactor/use webengine

this was a long-due refactor, that I had not finished properly because I didn't find the time to port the python/js bridge. it is working now, but it might be tricky to test:

• you need to install python-pyqt5.qtwebengine
• you need to put the index.html in place (since exposing the bridge object now it's not so straightforward as with qtwebkit; it needs to load the js library for qtchannel).
• there is some weird cache issue with chromium. i'm not sure of its extent, ie, if it affects only the devtools or the app itself.

note that with this refactor, if you launch the app with DEBUG=1, it exposes the devtools on localhost:8081

I hope when this is merged into master we can confirm that the "black screen" issues are gone #9165 (closed) - otherwise i'm out of ideas.