add java support for highchart functions showloading hideloading

[vaadin-bot]

Originally by bedinsky

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I'm using vaadin addon chart and I need to show a message when data is loading.

I have seen com.vaadin.addon.charts.model.Loading, that is the configuration to display loading message. Javadoc says:

The loading options control the appearance of the loading screen that covers
the plot area on chart operations. This screen only appears after an explicit
call to

chart.showLoading()

It is a utility for developers to communicate to the end user that something is going on, for example while
retrieving new data via an XHR connection. The "Loading..." text itself is
not part of this configuration object, but part of the lang
object.

chart.showLoading is a highchart API There's no Java API for calling the clientside showLoading method

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Imported from https://dev.vaadin.com/ issue #20176