Automated vcpkg, Win7+32bit support

• Prebuilts package is now generated from scratch using Github CI
• As a result, all libraries were updated to latest version available on vcpkg
• Now only boost components that are required for vcmi are included into the package (due to issues with boost-python on Win7/32 bit)
• Package now archived as .txz instead of .7z (hopefully will improve extraction times)
• Added / restored support for 32-bit builts
• VCPKG now targets Windows 7