- Queries in Explore have been string based
- This PR introduces the use of the DataQuery type to denote all queries handled in Explore
- Within Explore all handling of DataQueries is transparent
- Modifying DataQueries is left to the datasource
- Using `target` as variable names for DataQueries to be consistent with the rest of Grafana
- when instantiating a datasource, the datasource service checks if the
plugin module exports Explore components, and if so, attaches them to
the datasource
- Explore component makes all major internal pluggable from a datasource
`exploreComponents` property
- Moved Prometheus query field to promehteus datasource and registered
it as an exported Explore component
- Added new Start page for Explore, also exported from the datasource
