Windows package: improvements to the NSIS installer

— Reorganize the components: there is now a 64-bit group (with the MEX and
  MinGW), and similarly a 32-bit group. That makes it easy to cut down
  installation size when the user knows which one to choose. Add a description
  text that explains that.
— Dynare++ is no longer installed by default and is listed in the last position
— Remove the “Description” box in the components page, so that the components
  names are more visible