FluidSimulation3D example: fix bug, and change header guards

[ttnghia]

Hi @mosra,

I did some changes for the FluidSimulation example:

• Fixed a bug: the particle color cannot be changed.
• Fixed a bug: the light direction cannot be changed.
• I changed header guards of the *.h files, adding example name. This is necessary if we have different examples having files with the same file names.
• Fixed some warnings.
• Improve menu.

Also, the link in the example page (https://doc.magnum.graphics/magnum/examples-fluidsimulation3d.html):

Full source code is linked below and also available in the magnum-examples GitHub repository.

is wrong.

[mosra]

Merged as https://github.com/mosra/magnum-examples/compare/579a14cf46238501e648c7d5ce225edd73a99b9b...db48e1d899f54e742d9dd8f45bb7f439d3bd17f4 (without the #include changes), thanks!