Our "frame" concept very closely matches what the web specs call a
"browsing context", so let's rename it to that. :^)
The "main frame" becomes the "top-level browsing context",
and "sub-frames" are now "nested browsing contexts".
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
