Refactor optional feature handling to preserve signature of ui.plotly etc.

[falkoschindler]

Recently we checked in ui.py if certain dependencies can be imported. Otherwise a dummy function with args and kwargs was created to raise an ImportError once the element constructor is called.

Unfortunately this has the disadvantage of hiding the original signature in the IDE. VSCode seems to find the dummy function only, even if the dependency exists.

This PR moves such import checks into the respective UI elements, keeping ui.py nice and clean. Modules like plotly.py try to import dependencies and update globals.optional_features. If the import fails, the feature is not registered.

Only in the initializer we check if the feature is registered and raise an ImportError otherwise. This preserves the signature for the IDE and raises the exception exactly when needed.

But we need to make sure that the modules can be imported even if the dependency doesn't exist. In plotly.py I added from __future__ import annotations so that type annotations don't lead to runtime errors, even if there is no go.Figure. And in pyplot.py raising the ImportError is enough to prevent the remaining code from being executed.

[rodja]

Cool. It even looks cleaner 🧹