Allow data layers to render a filtered set of points

[abought]

Ticket: #196

Purpose

Implement a way for a data layer to "filter" its data and show only a subset of points. Two syntaxes are supported:

• Declarative, with built-in filtering operators (layout.filters = [ {field, operator, value}... ]). May be defined before initial render.
• Imperative, by defining an arbitrary filter function on the layer after the initial render is complete (layer.setFilter(func)). The user-defined function is a callable with the same arguments as array.filter. It is applied on each render.
  • Layouts must be JSON serializable; there is not a mechanism to define imperative functions before first render.

Summary of changes

• Remove unused filtering methods (addField, setElementStatusByFilters)
• Unify filtering syntax to be the same across labels and top level layouts. Breaking change: all now use {field, operator, value} (filters no longer declared as arrays)
• Add data-filtering syntax to scatter plot, genes, forest, etc. This will require further testing, as for some datasets the behavior is not very well defined.

TODO

• [x] Implement basic UI toolbar widget that allows quick filtering based on user-provided values
• [x] Add unit tests
• [x] Define behavior when user enters a filter, then clears value (esp for exact match)
• [x] Test on all layer types that support filtering functionality

[abought]

Screenshot of user-selectable filter functionality (in progress):

[abought]

Merging to facilitate testing, but open to further revisions before 0.12 final.