This makes splitters stand out visually so you can actually spot them.
Before this, you had to guess/know where they were, which was weird.
The look of the knurling is the same as GUI::ResizeCorner, to build on
the established visual language.
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 *
