Windows shell extensions: Use custom GUIDs via CMake and prepare MSI reg

Windows shell extensions: Use custom GUIDs via CMake and prepare MSI reg

Previously side by side installation with ownCloud or other NC custom builds would break the shell integration because the same GUIDs and registry keys were used.

Now we specify our custom GUIDs in NEXTCLOUD.cmake and use CMake to generate a header file and WiX (MSI) include file with these constants.

Note: Using generators like "guidgen" or "uuidgen" ensures that GUIDs are unique, as manual changes are not guaranteed to be.

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