refactored css

some improvements:
  * displaying a `horizontal ellipsis` within collapsed objects
    to indicate that they can be expanded
  * general cleanup.
    much of the css was previously taken as-is from `jsonTreeViewer`;
    gave it a closer look and made a few tweaks.

the ultimate intent is to migrate some rules into user preferences,
which will allow tighter control over the presentation of the data.

these rules will (initially) include:
  * font-family
  * font-size
  * line-height
  * padding around the body
  * width of indentation for expanded children
    (note: 1.5em is required to display the expand/collapse button)

this commit still hard-codes these values within the css file.
however, the rules have been segregated
and are ready to be migrated elsewhere.