This patch replaces the concept of fixed resizees with opportunistic
ones which use the new SpecialDimension::OpportunisticGrow UISize.
This lets us simplify splitter resize code and take advantage of
the layout system's automatic calculations for minimum size and
expansion. Functionally the same as before, but fixes Splitter's
unintended ability to grow window size.
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.
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 *
