Minor tweaks to Geocoder

This prevents the Geocoder widget from both allowing editing of text while searching and also prevents submitting of the text "Searching..." while already geocoding.

Fixes #2087
Fix widget error handling

Fix #2095 by making sure that exceptions generated by onTick callbacks are handled by the widget.
Remove superflous messaging from errors that are most likely not WebGL related.
Merge pull request #2153 from AnalyticalGraphicsInc/geocoder-searching

Minor tweaks to Geocoder
Merge pull request #2154 from AnalyticalGraphicsInc/ontick-errors

Fix widget error handling
`foreachLoadedTile` should not enumerate tiles in the START state.

Such tiles aren't "really" loaded.
Update CHANGES.md.
Add a spec for forEachLoadedTile change.
Merge pull request #2155 from AnalyticalGraphicsInc/forEachLoadedTile

Fix a bug in `QuadtreePrimitive.forEachLoadedTile`
Initial submit of maki icon set.

I'm actually doing this on a separate branch to make the code that uses
it actually reviewable on GitHub.
Merge branch 'maki-icons' into geojson-simplestyle