* Added test case of layer delete bug
* Fixed crash in `PenTool`
Updated the document `MessageHandler` to cancel all active tools if the
layer is deleted. This prevents the tools from crashing due to the
layer being pulled from under them.
* Moved Abort into pre-graphene DeleteLayer message
* Renamed test case for clarity
* Moved tool crash tests to the `tools` module
* Added `test-case` to the dev dependencies
* Added crash test case for all tools
* Ran cargo fmt
Co-authored-by: otdavies <oliver@psyfer.io>
Co-authored-by: Keavon Chambers <keavon@keavon.com>
