CPackWiX: allow customization of generated WiX sources

Added a new variable CPACK_WIX_PATCH_FILE that users can point at an
XML patch file. Fragments defined within the patch file will be inserted
at supported insertion points (currently Component, File and Directory).
Merge topic 'wix-fragment-injection' into next

8632233 CPackWiX: allow customization of generated WiX sources