Add CSS class to widget when fetching data from backend

[Alex-Sichkar]

When a widget fetches data from the backend (e.g., when switching between time periods in a calendar), add a CSS class to indicate that the operation is in progress, so it can be styled accordingly. This is especially useful in cases of slow server responses.

[jrief]

What kind of proposal do you have for this?

[Alex-Sichkar]

Simply add something like the df-loading CSS class to the widget when it's fetching data from the backend, and remove it once done.