Remove the barely used configurable program name.

It's hardcoded in 90% of the places anyway.
Factor out icon installation into the corresponding sub-directory.
Reorder some stuff to have related code next to each other.
Move API docs generation entirely to its own directory.
Remove the manual DISABLE_FIND_PACKAGE handling.

Not needed, works all by itself.
Move man page generation into the docs/ sub-directory.
Port to CMake built-in FeatureSummary.
Move this option to config.h to be consistent with all other defines.
Move stuff out of the external dependency section that doesn't belong there.
Port to new way of specifying the link interface libraries.