Advanced data explorer

[loicknuchel]

Remaining TODO:

• [ ] Need column type in DbQuery.exploreColumn
• [ ] Upgrade Vectual & use if for pie chart & bar chart
• [ ] Adapt to other db than Postgres & MySQL (DbQuery + connectors: column ref)
• [ ] Improve query result stats (known types such as date, more charts & stats)
• [ ] Issue when open data explorer from erd for the first time: everything goes up :/
• [ ] Many other things ^^ (see DataExplorer.elm TODOs)