Adding GraphicsMagick thumbnailer based on ImageMagick's

[symac]

I had noticed some months ago (#1358) that it was impossible to generate thumbnail when using graphicsmagick instead of imagemagick, even if the two tools are almot identicals. The reason is that -alpha remove is not a recognized parameter for graphicsmagick. Until now I had bypassed this by commenting the line in imagemagick thumbnailer, but per @Daniel-KM suggestion on the issue, I have now created a new Thumbnailer.

Rather than rewriting it from scratch as the only difference with ImageMagick is only the alpha remove option, I have extended the ImageMagick class and added a specific function to deal with arguments and remove the faulty one for GraphicsMagick.