GeoJSON loading performance improvements

1. Allow `ConstantProperty` to optionally return a reference for complex types rather than always cloning.
2. Have `GeoJsonDataSource` use a canvas object for `BillboardGraphics.image` instead of calling `toDataURL` on it.
3. Modify `Billboard.image` property to use the `id` property when a canvas element is assigned.
4. Have `PinBuilder` create a unique id for each pin canvas.
Allow ConstantProperty to be used with HTML elements.

Normally `ConstantProperty` requires `clone` and `equal` functions
for non basic types, but since `HTMLElement` instances support
reference equality; that's not needed in this case.
Update comments and CHANGES

Also fixed deprecated parameter used in viewerDragDropMixin.
Merge remote-tracking branch 'origin/master' into geojson-load-performance

Conflicts:
	CHANGES.md
Merge remote-tracking branch 'origin/master' into geojson-load-performance

Conflicts:
	CHANGES.md
Merge remote-tracking branch 'origin/master' into geojson-load-performance

Conflicts:
	CHANGES.md
Changes after review.

Remove use of canvas.id from `Billboard`, but still allow it for `BillboardVisualizer`
Merge remote-tracking branch 'origin/master' into geojson-load-performance

Conflicts:
	CHANGES.md
Fix CHANGES after merge.
Merge remote-tracking branch 'origin/master' into geojson-load-performance
Move changes to 1.5
Merge remote-tracking branch 'origin/master' into geojson-load-performance

Conflicts:
	CHANGES.md
Changes after review

Don't rely on canvas id as a billboard id in `BillboardVisualizer`; which
was originally put in to optimize GeoJSON loading.  Instead have
GeoJSONDataSource cache the resulting dataURL so that conversion is only
done once.
Merge remote-tracking branch 'origin/master' into geojson-load-performance

Conflicts:
	CHANGES.md
Merge remote-tracking branch 'origin/master' into geojson-load-performance

Conflicts:
	CHANGES.md
Tweak CHANGES.md
Merge pull request #2278 from AnalyticalGraphicsInc/geojson-load-performance

GeoJSON load performance
Merge remote-tracking branch 'origin/master' into font-outlines