MacOSX and Windows platform enhancements and installers

Hi, 

this series of commits fixes various small issues in CMake machinery and adds all needed ressources to build both a NSIS installer on Windows and a Drag-and-drop installer on MacOSX (both tested here, they work).

I have also amended and created documentation to make sure others can compile Colobot for their purposes as well.

        make package

should build:
* a NSIS package (given that there's a makensis in PATH) on Windows, including modest branding.
* a Drag-and-dropp package on MacOSX.

They both install to system paths and make Colobot "look" much more integrated.

Please consider. :-)

Cheers, OdyX