New demo market

[texodus]

Adds a new Perspective examples market, a JS only reimagining of #1277. This multi-process example implements order clearing, expiration and pricing as Perspective queries.

Demo here!

Other fixes in this branch:

• Adds min and max aggregate functions. Unlike low and high aggregates, these are always recalculated, which means they track the range for the current dataset (as opposed to all data entries to ever exist in the dataset, even if they've been updated or removed).
• Fixed bug which printed the string "null" over null label points in "X/Y Scatter" charts.
• Fixed Candlestick and OHLC charts to allow <4 columns, where the missing fields are omitted (in the case of high/low) or synthesized (as the next open in the case of close).
• Fix charts to only grow extents on update(), which leads to a less janky visual experience when the dataset changes range quickly. The range is reset on any interaction.
• Fix load() to not overwrite default column on 2nd+ calls if they have already been set.
• Fix drag & draop implementation on iOS (remove mobile-drag-drop polyfill and adapt native HTML5 drag/drop behavior to work on iOS).
• Add query_columns() method to Table, which emulates view() + to_columns() + delete() in a single event loop dispatch.