This adds a tooltip to all the slider properties showing their
current value. Previously there was no indication of what
value they had. Also rename the SprayTool property 'thickness' to
'size' like BrushTool calls it.
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 *
