Reset selection source upon clause eviction

[jheer]

Selections with a single resolution strategy will evict an existing clause upon updates. This PR adds a mechanism for reporting this eviction back to a clause source: if a source has a reset() method, it will be invoked upon clause eviction.

Both the menu and search widgets and the Interval1D and Interval2D interactors now have reset() methods that clear their interactive state upon selection clause eviction.

[jheer]

@domoritz This provides the "single brush" functionality discussed under #35.

[manzt]

Is there a mechanism to batch .reset() calls? For instance, I'd like to wire up a button to efficiently "Reset all" 1DIntervals.