Even though they are called content_margins,
they are actually only ever used to determine where
a Widget is supposed to be grabbable.
So all methods and members are renamed accordingly.
Instead of computing the grabbable areas on the fly in mouse event
handlers, we now figure out which parts of the splitter are grabbable
when something moves around, and then remember it.
This makes the code a lot easier to follow.
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 *
