Days 31/32: More web elements

Apart from fixing some QtWebEngine bugs, yesterday and today I made some big progress with the web element code. I successfully split the web element code into a generic and a QtWebKit part, and then did a first implementation of the web element code for QtWebEngine, which made :navigate prev and :navigate forward work.

After implementing some more missing web element stubs (and adding a missing .lower() call), I got :open-editor to work as well.

I then noticed the javascript linter qutebrowser is using (eslint) didn't actually report anything. I tracked this down to a v1.0.0 release which turns off all rules by default, which I somehow missed. After reconfiguring it, I had more useful output and cleaned up some style issues.

Then I properly modularized the javascript code and did some other cleanups to ensure it doesn't remain in its initial proof-of-concept state for any longer.

By chance I noticed that flake8 (one of the Python linters qutebrowser is using) also didn't do much anymore, which was due to me not passing a file path to it. I fixed that and also cleaned up some issues which went unnoticed in the meantime.

I planned to work on the hint drawing code, but since various pull requests related to hints were opened today, I decided to postpone that to after they are merged.