Allow docdir to be defined at build-time
- Simplify by using GNUInstallDirs module
- Move html documentation into an html subdirectory
Hardcoding docdir is a problem for Gentoo Linux where the standard is to install a single directory in /usr/share/doc/ named with the Gentoo package version number.
Changing to GNUInstallDirs affects more than just docdir but I think it makes sense to do it all at once - otherwise it would mean creating one solution just to modify it straight away.
Default behaviour remains the same (except the html subdirectory of course).
I believe using GNUInstallDirs makes the CMakeLists.txt files more maintainable.
I haven't tested the gtk-update-icon-cache commands. I'll raise an issue about why not.