- All C++ code has been moved to a new `cpp` folder.
- C++ commands now reside in `cpp/cmd`.
- We now build a single (shared by default) library that packages all 
non-gui code previously in `core` and `src` (now in `cpp/lib`).
- All gui code in `cpp/lib/gui` is still built separately as before.
- Version matching logic for library and executables has been updated.

By setting, CMAKE_FIND_FRAMEWORK=LAST we instruct CMake to not look
for header files inside /Library/Frameworks as a first search path.
This can cause issue when frameworks like Mono are present on the system,
resulting in CMake unable to find the homebrew library headers.

The strategy now matches what we do to get the library version.

This ensures that the version string of library and commands live in
separate binary objects. The version matching logic has been moved from
app.cpp to command.h to accomodate for this.

- `cpp/lib` is now split between `core` and `gui`, the former contains all code previously under `core` and `src`
while the latter contains only GUI code for `mrview` and `shview`

- `mrtrix-headless` has been renamed to `mrtrix-core`

This is to avoid conflicts with system headers.

We ensure that glutils.h is included prior to gl_core_3_3.h since it
indirectly includes Qt's OpenGL headers (which breaks the build).