Align tools to use similar UI/UX

[forman]

The recent tools of xcube Viewer follow mostly unaligned concepts, placements, stylings, and operations. They should use a single, aligned UI/UX concept throughout the app so that it is more intuitive, obvious, and comprehensive for users.

Most tools are based on the current selection of model elements: selection of dataset, variable or variables, timestamp or range, place or geometry or multiple of them.

The Statistics tool could serve as a refence here. It shows the current selection and what's missing to let user add a new statistic result. Here a place selection is missing therefore the add button is still disabled. (A tooltip text should explain the situation.)

Once the place is selected, the add button is available:

Apply the same concept to other panels which currently use other concepts and UIs:

• Time-Series - Also: panel should be disabled and show a message if no time dimension available

• Volumes

• 2D Histogram (Demo) - (1) see note regarding server-side extensions below

Adding a new computed variable, a time series, or new statistics can also be done from convenient tool buttons next to the variable selector:

Preferably all the tools should have such "add"-shortcuts in the toolbar.

(1) Note regarding server-side extensions: consider providing a Python component for the purpose of forcing a certain selection, then adding items, and later removing items. Will create a separate feature request for that. See also #452, #494.