Improve CMake usage

[ledocc]

I propose to improve CMake usage in many way:

• spit CMakeLists.txt and dispatch code in sub-directory. In the top level CMakeLists.txt, lets project initialization, project options, headers/function checks, and other global stuff. In common, client_example, examples, libvncclient, libvncserver, and test folders, move cmake code related to each folder.
• use target_... function instead of global function
• replace deprecated functions like install_files
• add cmake config support to easily import LibVNCServer in other project

all of this will be done on top of #309

thought?