Actually, custom label are not allowed, it directly uses the provided by
the query result as the target/alias
This feature allows to:
- Split YLabel to store its partials and reuse them on link and extra tooltip field
- Override YLabel with splitted and custom values
- Added Extra Tooltip to show metadata to the user on each serie
- rename variables for URL template, use templateSrv to render URL links
- fix visibility of tooltip items and url template help popup whe the light theme is used
- rework Tooltip settings pane, organize tooltip editor fields
- rename properties in panel config, implement migration logic for legacy configuration
- close button for the frozen tooltip
- use relative positioning for proper page scrolling