WIP: Allow to select poppler's Arthur (=Qt) render backend for pdfs
The Arthur backend is much less mature than the Splash backend, but it seems to beat Splash in a few particular situations. For example, for me it solves the problems with thin-line rendering reported in https://bugs.kde.org/show_bug.cgi?id=416796.
The patch is WIP because one manual reload (F5) is needed to activate the newly selected backend. I need to figure out how to automate this.
I am tempted to complete avoid the word "Arthur" in the GUI. That expression does not seem to be used at all anymore outside of poppler.