Bug fixes for perspective.md and iOS

[texodus]

Fixes bugs:

• Fixes perspective.md to generate a well-formed response when the underlying Table is empty. perspective itself returns an empty object in this case, though arguably it should provide the column_header keys, this is still potentially logically empty in the case of column_pivots.
• Fixes a CSS quirk which makes interaction events apply ::select pseudoclass and styling.
• Fixes a regression in iOS which causes virtual scrolling to "bounce" along with the headers in a fairly unattractive way, at the expense of scroll inertia. We may need to revisit this if scroll inertia is a popular feature ...