fix compilation with QtWebEngine

[cyplo]

Hi !
I find mindforger very useful and I help packaging it for NixOS.
We're trying to deprecate QtWebKit across all NixOS packages.
As part of that work I found that mindforger accepts "CONFIG+=mfwebengine" and then tries to compile with QtWebEngine.
When I ran the build I found that the project does not fully successfully compile with that option though.
This PR is an attempt to fix that - the resulting binary works very well on NixOS, I however haven't tested it on Windows nor MacOS.
I think I shouldn't have broken these targets, just by reading the the code, but would probably be best to test on those ?

For reference - here's the PR I'm trying to land to change the NixOS package definition - it includes these changes as a patch for now, but the reviewers suggested upstreaming first, hence my PR here :)

Hope this helps, please let me know what you think :)

If this makes sense, I would like to open another PR for the second patch that makes the output directories relocatable, but didn't want to spam with PRs without opening up a conversation.

[godlike64]

I just ran into this issue as I was trying to compile Mindforger on my Gentoo box to try it out, but on Gentoo QtWebkit has already been removed. I cloned the branch with the commit that addresses this, and I was able to compile and run MF properly (still haven't tested it out fully yet). I used:

qmake CONFIG+=mfwebengine -r mindforger.pro

Thanks for the patch!

[dvorka]

@cyplo thank you for building MindForger on NixOS and PR contribution! I applied PR manually to both dev/1.54.0 39cb330 (upcoming release) and master 89467aa manually.

@godlike64 thank you for building MindForger on Gentoo and interest in MindForger! 👍

Closing PR as applied.

[cyplo]

thank you ! :)