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.