After closing the last open ImageEditor, selecting a color would try to
dereference it causing a crash. Instead make set_image_editor() take a
pointer to it and set it to nullptr when closing the last tab like we
do with LayerListWidget and LayerPropertiesWidget.
Previously, if you wanted to use a custom color, the only way to
do so was to first Ctrl+click on one of the pallette colors, which
would just change that palette item. Then, you would need to
manually click on that color.
Now, you can just click on the preview of the primary/secondary
color to open up the picker and only temporarily use the new color
without affecting the palette at all.
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 *
