Napari-pyclesperanto-assistant

New features:
* Select-GPU dialog when starting the Assistant
* Support for 4D-data by processing 3D stacks individually and updating when the user changes the time point slider
* Function for turing a 3D image into a 2D+t stack
* Renamed categories, e.g. "Background removal" -> "Remove background"
* Reorganized categories: Better differentiation between label neighbor filters, label measurements and label-image measurements

See also
* https://github.com/clEsperanto/pyclesperanto_prototype/blob/master/docs/release_notes.md#094---july-3rd-2021

See also https://github.com/clEsperanto/pyclesperanto_prototype/blob/master/docs/release_notes.md#092---june-26th-2021

New features
* User interface for measuring label statistics (in the menu)

See also:
* https://github.com/clEsperanto/pyclesperanto_prototype/blob/master/docs/release_notes.md#090---may-16th-2021

New features
* Show multiple dock widgets in napari 0.4.8
* Moved menu entries to buttons in the assistant widget
* Use dask under the hood to manage execution order of operations
* New and more flexible user interfaces for the button grid
* Push/pull images to/from GPU memory were minimized bringing improved performance, napari can have OCLArrays as layers since [pyclesperanto 0.8.0](https://github.com/clEsperanto/pyclesperanto_prototype/blob/master/docs/release_notes.md#080---apr-22nd-2021)

Bug fixes
* Operations are no longer executed twice when added
* Some categories didn't show all parameters which were needed to configure the operations

Backwards compatiblity breaking changes
* Improved, simplified and easier extensible backend for categories
* New code generators based on jupytext

Special big thanks to Talley Lambert who made most of these!

Bug fixes
* The user interface was confused when multiple operations of the same kind were executed. Thus, better handling of stateful operations attached to layers was implemented. Big thanks to Talley Lambert for suggesting how to fix that.

Miscellaneous
* A test was added to simulate a complex workflow executed in the CI.