CPack/DragNDrop: Support RTF licenses
I know less about DMG's SLA than the other authors of this code, but I learned this useful nugget from https://github.com/andreyvit/create-dmg/blob/master/support/dmg-license.py#L95
I don't know how to automate a test. When mounting a .dmg on the command line it bypasses the license popup. So this was tested by hand, both the CPACK_RESOURCE_FILE_LICENSE
and CPACK_DMG_SLA_DIR
variants.
The code prefers the .txt over .rtf for backwards compatibility, in case someone happens to have both files present their build won't suddenly start picking up the other one.
The .txt files are blindly treated as plaintext. I didn't think there was much value in probing the magic bytes to test for RTF-ness. It's easy and arguably more clear if RTF files use the .rtf extension. Similarly non-RTF files with a .rtf extension feels wrong.