Mb/53 - Add ground transparency slider to basemap gallery widget

[MatteoBerg]

This PR resolves #53, adding the ability to adjust basemap/ground transparency which will allow users to view objects underground.

I imitated styling of the ground transparency slider in the Central Basin 3D Scene - v1.3 beta scene shown here:

Here is a demo showing the added functionality matching the above example scene:

basemap-transparency.webm

[tbock]

It looks like you are not setting the slider to the current value of the basemap's transparency when it loads. Can you confirm/fix @MatteoBerg

[MatteoBerg]

Fix slider start value

Changed slider start value from 1 to map.ground.opacity value when slider is created.

[tbock]

Slider is wrong after selecting a slide then opening the base map gallery again. The slider is still at old value not the value of the current ground opacity. @MatteoBerg

[MatteoBerg]

Feature is included in branch mb/47, PR #104