Removed FeatureLayer from QueryLayer widget.

Added code to keep track of selected query layers in `Search` widget.
Closes #26.
Changed widget property names to match queryLayers.json file.
Group query layers by header.

Closes #18.

Plus some layout improvements including making the entire title bar for a pane clickable instead of just the text.
Merge pull request #29 from agrc/query-layer

Query layer improvements