Enhancements for Layer addition from metadata (via FindLayers, i.e. CSW)

[justb4]

This issue complements issue #230. That issue deals with enhancements to Layer addition via "Capabilities", i.e. the "AddLayers" GXP widget. This new issue deals with "FindLayers" related widgets, i.e. adding new Layers via CSW. As with #230 the work is performed in the context of the Heron Mapping Client (http://heron-mc.org) whose users have requested "Catalogue" features.
The related Heron issue is https://code.google.com/p/geoext-viewer/issues/detail?id=400.
All code changes are made to a GXP fork maintained at: https://github.com/justb4/gxp. This repo is then included in the Heron SVN repo via svn:externals and its build of gxp.min.js.

I know GXP is now used as an SDK client template within the Boundless OpenGeo Suite, and possibly only operates in that context (?), but several planned enhancements and bugfixes may be useful.

Planned is the following:

• UI enhancements: show more MD info than just title and part of abstract: date, full abstract, link to full MD record etc.
• CSW interactions:
  • BoundingBox/Extent may be missing in CSW record (causes GXP errors)
  • BoundingBox/Extent has wrong SRS (at least in Dutch national georegister)
  • CSW rec may have multiple <dc:URI protocol="OGC:WMS" elements
  • not all WMSs use dc:URI name attribute as layer name
  • many quirks with custom and ESRI-based WMSs: no GetLegendGraphic

Optional:

• allow WMTS addition
• allow WFS addition
• allow Atom (download addition)

The canonical Heron example will be at http://lib.heron-mc.org/heron/latest/examples/catalognl/
using CSW from the Dutch National SDI (GeoNetwork) at http://www.nationaalgeoregister.nl (which has quite some challenging/polluted CSW data!)

A general problem is how to deal with cross-domain/proxy issues: not only may a W*S domain not be configured in the proxy, not an issue for raster-based WMS/WMTS, but e.g. GetFeatureInfo in WMS may not work.