Update wkhtmltopdf libraries to 0.12.5 and fix a callback was made on a garbage collected delegate bug

[rndusr0]

Great work on this, thank you, but not sure if this project is now dead? Thought I'd make another PR since my last one anyway, with a couple of updates and fixes:

• Updated macOS and Linux wkhtmltopdf to 0.12.5 in addition to Windows in the last PR, removed 0.12.4 and updated nuget build
• Fixed (I think!) 'A callback was made on a garbage collected delegate' issue - PdfTools should now hold a reference to any delegates in managed code so the GC doesn't collect them, thought this was probably best done in PdfTools rather than in the BasicConverter, so any other consumers of PdfTools shouldn't have to worry about this issue, but I could be wrong...
• Added licence link to nuget package meta-data

[rndusr0]

If it's useful to anyone, I've published this as a nuget package at https://www.nuget.org/packages/RndUsr0.DinkToPdf/

It's only been tested on Windows, but any feedback gratefully received...

[jwillmer]

Thanks for updating this library! Your version works very well for me. I only get Qt: Could not initialize OLE (error 80010106) in the output window after I convert a file - conversion is successful. Can you tell me if I can ignore this error or what it means?

[alexander-nesterenko-kas]

@rndusr0 When do you remove delegates from the list? Is it possible leaks?

[welldtr]

Has this solution been merged or reproduced in latest releases? I've been facing this problem on 0.12.6-1 version