Color palettes can now be stored in and read from files. The default
palette will be read from `/res/color-palettes/default.palette`
instead of being hard-coded in PaletteWidget.
The file format is one color per line, in any format that can be
understood by `Gfx::Color::from_string`.
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 *
