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

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- 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

React panels query editor fixes

Panel focus in react

Moved query manipulations from metrics controller to metrics tab so they are more easily shared between angular and react panels

minor changes to react panels

Panel size refactor

fixed singlestat guage ceneterd dot rendering issue