Some tools (e.g. ZoomTool) doesn't need layer to work. This commit
makes mouse events fire even if there is no layer. This fixes
a bug that ZoomTool didn't work when there is no layers.
This commit adds a Tool::MouseEvent struct, which contains events that
may be needed by tools: layer-relative, image-relative and raw (editor-
relative) event.
The raw event is used by ZoomTool to properly pan the view. This fixes
a bug which caused image to snap out of sight.
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 *
