formhandler: view option for charts

With _by option #71 (closed) added.

User will have flexibility to construct large number of usecases from formhandler interface. Currently, table view is the default.

It would be useful to have view dropdown on options to select chart type.

After selecting a chart, ex scatterplot, chart options like x, y, size, color, will be automatically selected from dimensions and metrics in naive order.
User can modify chart options from a modal popup, to select x, y etc.
url parameters preserve chart options
For now, charts with minimal/flat options will be supported
End URL should be shareable/embeddable

Edited Jul 02, 2018 by Pratap Vardhan

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