Make CPACK Generator for Windows a cache variable

Currently the generator is set to NSIS, if windows is used with a normal variable.

We should use a cache variable instead, so the user is still able to change the generator per CLI of cpack. This could be neccessary, if one would have the 64bit version of NSIS, whose generator is NSIS64, or one would want a simple ZIP for distribution to people without admin access on their machines.

Change:

    set(CPACK_GENERATOR "NSIS")

To

    set(CPACK_GENERATOR "NSIS" CACHE STRING "Generator used by CPack")

(While we're at it, we could add a note, that all libraries for PeTrack need to be accessible in the path, i.e. petrack.exe should be startable with just a double click, for fixup_bundle and otools)