CMake: add NSIS generator settings

* Add icon to VCMI_client and VCMI_launcher for cmake/windows build.
* Add ability to build NSIS .exe installers.

How-to build:
* .exe - `cpack -G NSIS`
* .zip - `cpack -G ZIP`

Output format:
vcmi-x.xx-win32.(exe|zip)

Example:
vcmi-0.96-win32.exe