SVG Path Helper

This adds a new SVG helper class, and deletes all the SVG files from Widgets/Images.  Currently it is wired up to the Home button, Scene mode picker & dropdown, and the FullScreen button, and its effects have influenced the BaseLayerPicker as well.

The above-mentioned widgets are now "flat" style (a minor change, but it makes them easier to theme, and removes pesky SVG gradients).  The widgets are also now "button" elements that capture the input focus if you TAB around the page.

If you inspect one of the widgets, you'll find the class `cesium-widget-button` with a `fill` color on it.  You can change this color to affect all the SVG icons using the new system.  Previously this needed to be done by hand-editing the SVG files in the Images folder.  The new widgets can be themed in the browser, and can have effects like `:hover` influence internal colors, which also was not possible previously (just a `box-shadow` was done).

My hope is to use `SvgPath` for additional little graphics on various upcoming UIs.  One could imagine a search box, a clock icon, maybe menu or settings, and all kinds of stuff.  The Animation Widget could eventually make use of this too, and the Timeline, but they don't yet.