Windows MSI: Add Upgrade Code and migration tool build options to NEXTCLOUD.cmake

Windows MSI: Add Upgrade Code and migration tool build options to NEXTCLOUD.cmake

The Upgrade Code is a GUID (specified without brackets) for the MSI package to allow Windows Installer identify existing installations.

New build options (default: OFF):
- BUILD_WIN_MSI: Build all MSI scripts and a required helper DLL (to uninstall NSIS legacy installations and remove Explorer Navigation Pane entries)
- BUILD_WIN_TOOLS: Build additional migration tools (currently NCNavRemove, a stand-alone tool for Explorer entries removal)

The helper DLL and migration tools are set to be statically linked and optimized for binary size.

Signed-off-by: Michael Schuster <michael@schuster.ms>