This patch adds a GUI::TabWidget to the main UI and allows having
multiple images open at the same time.
Some of the changes here are a bit hackish and mechanical and there's
still code around that needs more work to fit better in the new world.
One nice side-effect of this change is that ImageEditor now always
has one Image associated with it, and it never changes.
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 *
