Different thread highlights between widgets lead to different
visual weights between splitters, even when they have the same
width or height. This means some splitters look best at odd
sizes while others even. This sets the default spacing to the
most commonly used, depending on orientation, and adjusts
spacing for a few apps based on the new paint rect.
The most consistent look across apps requires some manual
tweaking occassionally. Knurlheads, use your discretion!
Splitters could be resized in such an order that all their remaining
children were fixed size, leading to unfillable gaps on resize events.
HackStudio and TextEditor already had logic to handle this edge case,
so this patch factors it into a general solution for all Splitters.
At least one widget is now guaranteed to be resizeable after a child
is removed.
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.
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 *
