- extends handlers for panel menu and keypress 'x'
- in a mixed-datasource panel finds first datasource that supports
  explore and collects its targets
- passes those targets to the found datasource to be serialized for
  explore state
- removed `supportMetrics` and `supportsExplore`
- use datasource metadata instead (set in plugin.json)
- Use angular timeout to wrap url change for explore jump
- Extract getExploreUrl into core/utils/explore