Implement 'WorldWind Viewer' into WWSK

[Beak-man]

Deriving from the work on #90, implement a full-featured Web WorldWind-based asset viewer inside WWSK. This will complement (functionally speaking it may substitute) the current 'Layer Preview' features that were inherited from GeoServer, enabling WWSK to operate as a complete end-to-end solution for end users.

Two routes to implement the viewer should be considered:

~- Making it part of the rest of WWSK's admin web panel (this is, a web application built under Apache Wicket/FreeMarker)~

• Building a complete separate web application, with its toolset at our discretion

• [x] Layer Manager model
• [x] Layer Manager UI
• [ ] Server-side settings
  • [x] Viewer menu link
  • [ ] Default layers
  • [ ] WMS Version
  • [ ] Endpoint (WMS or GWC)
  • [x] Vendor params (e.g., tiled=true)
• [x] Session-based settings (client)
• [x] URL based initialization
• [x] Zoom to layer

[emxsys]

Added a high-level use case diagram to the epic.

[emxsys]

Added a prototype viewer--the Explorer--to WWSK. The Explorer includes a layer manager that exposes the layers found in the GeoServer's WMS GetCapabilites document in a sortable list that includes persistence between sessions. The Explorer includes the ability to "zoom" to a selected layer. The Explorer includes the capability to generate and copy a URL containing the active layers to the clipboard as the basis for a "bookmark" or sharable link.