Draft: library variant that works without QtGui
I added aliases for QPalette, QColor, and QRgb with the bare minimum of what is used as a member function. The Palette constructor is completely different; it takes the background and selection colors. This is controlled via the KSYNTAXHIGHLIGHTING_NOGUI macro. For now, ifdef are commented out, so it is always enabled...
No tests have been done yet on Color, and Color::fromString() has not been implemented.
I added KF6SyntaxHighlightingNoGui, which must be compiled explicitly and exists in parallel with KF6SyntaxHighlighting. Nothing about installation for the moment. I wonder what would be best? Have both libraries installable at the same time and the NoGui option that keeps only one, or have only one library at a time? I also suppose that the headers would have to be installed in two different trees?
I also wonder about DefinitionDownloader, which I've never really found practical (no control over what is downloaded, where, how, and does not respect NO_STANDARD_PATHS). I guess that apart from kate-syntax-highlighter, no one uses it. I'm tempted to remove it from the NoGui version or add an option.