[skip ci]

panel-header: Move the panel description/links/error container outside of panel header to not interfere with the react-grid stuff

panel-header: Make it possible to style the reference element and fix so panel description looks good

panel-header: Updates for the new react-popper api and make it possible to hover the tooltip popper without it closing

panel-header: Bump @types/react, @types/react-dom, react, react-dom, react-popper to latest versions

[skip ci]

[skip ci]

[skip ci]

- get rid of drop shadow
- get rid of top border
- add min height for better no-data placement
- add margins

Datasource selection triggers a connection test, on success
the DS is set in the Explore state. If the test takes long and user
selects a different DS, and just after that the first test succeeds,
then the first DS overwrites the state.

* when test returns check if datasource is still the requested one

moved slider into label to make it clickable, styled slider in dark and light theme, created variables for slider

12759 React-panel: description and links