Modernize CMake build

Currently shiboken2, pyside2 and pyside2-tools are separate CMake projects, which are built and installed separately.

This issue aims to:
1) Provide a super project "CMake project" which would build the three projects above at once. The old way of building each project separately still remains.

Benefits: Usage of Qt Creator's CMake integration for easier development of the project. Or any other's IDE CMake integration for that matter.

2) Improving the build system to use new target-based CMake constructs, as opposed to add_directory-based constructs. E.g. think of propagation of compiler and linker flags when using shiboken2 while creating PySide2 modules.

Benefits: Clearer separation of concerns, simpler maintenance of the build system.

3) Provide CMake and python config files, so that end users can build bindings for their own libraries either via CMake or some as-yet-not-defined python process. Point 2 is a prerequisite for point 3.

Benefits: Provide users ability to create their own bindings, in a user friendly way.