Start of an Imagery widget.

Still a ton of work to be done.
Merge branch 'sceneModeWidget' into imageryWidget
Start experimenting with layout.
Turn imagery widget layout into a dropdown with labeled icons.
Use a trasition to fade in dropdown.
Merge branch 'sceneModeWidget' into imageryWidget
CSS tweaks

Also fix the imgery dropdown fade-out transition.
Cleanup and tweaks to the ImageryWidget

Still have to write unit tests.
Add scrolling to the ImageryWidget drop-down.

Also add a media-query on CesiumViewer to change the max-height of the dropdown on shorter screens.
Better handling of customized styles.
Merge branch 'sceneModeWidget' into imageryWidget
Rename `ImageryPicker` to `BaseLayerPicker`

Eventually, I'll break out the dropDown panel from the picker into it's own widget.
Move ImageryProvider icons into their own sub-folder, also remove two unused files.
Merge branch 'sceneModeWidget' into imageryWidget

Conflicts:
	Source/Widgets/Dojo/CesiumViewerWidget.css
	Source/Widgets/Dojo/CesiumViewerWidget.js
	Source/Widgets/lighter.css
Merge branch 'sceneModeWidget' into imageryWidget
Start of BaseLayerPicker specs.
Minor cleanup, shell for specs.
Decided I like the 32x32 imagery icon better.
Bring BaseLayerPicker specs up to 100%
Merge branch 'sceneModeWidget' into imageryWidget