Ease development of server-side viewer panels

[forman]

Changes:

• Added developer reference docs generated from derived state's property descriptions
• New module ext that deals solely with the server-side contributions
• Introduced new derived state for server-side contributions
• Updated the demo panel "2D Histogram", it is still lacking some useful features like showing progress, disablement, and some info text

To verify

• make sure dev reference works: extensions and derived state is documented
• make sure the demo panels work, please leave comments with possible enhancements or ideas for other panels

See xcube-dev/xcube#1092

[clarasb]

The feature work fine.

As requested, here are some ideas for enhancements:

• Adding the place name to the plot, otherwise there is no obvious/visual connection between the plot in the panel and the place.
• When creating new places, the Viewer automatically jumps to the 'Time Series' panel. This is a bit confusing if the user is in the '2D-Histogramm' panel and wants to create a new place for a new 2D-Histogramm.
• In the other panels, the control features (buttons etc.) are placed at the top of the panel. For consistency, the controls in this panel could also be placed at the top instead of the bottom.

Another panel idea: a simple histogram only for one variable

(tested from a user-perspective)

[forman]

As requested, here are some ideas for enhancements

Thanks! Once we have a new release, let's please add those as a feature request.

[clarasb]

@forman done, see #452