Cytopix

- feat: add buttons for controlling annotation tools
- feat: add Otsu segmenter for PNG files
- feat: allow to disable segmenter
- ref: add method `SegmentationSession.set_segmenter`

- setup: remove unused code from windows spec file

- build: do not pin mahotas

- build: add Windows and macOS installers

- fix: disable native menubar on macOS

- setup: move to src layout
- docs: properly employ license GPL-3.0-or-later