Fix #1194: Style selector for wms

[offtherailz]

This introduces support for style selection for WMS layers:

• Lazy loading of the styles list when open
  
• The styles list will be saved into the map, and can be refreshed using the refresh button.
  
• in case of error due to timeout or error it display a message
• automatically tries to query the single layer capability if it finds /geoserver/ in the path (the layer has a specific capabilitiesURL or the /geoserver/ path has been recognized.
• You can edit manually the value
  

The only issue can happen when:

• single capabilities are not exposed in GeoServer
• capabilities don't follows the schema for WMS 1.1.1

In those cases an error will appear, but you can anyway edit manually the style name.

[coveralls]

Coverage increased (+0.2%) to 79.963% when pulling 334bc30 on offtherailz:styles_wms into b956654 on geosolutions-it:master.

[coveralls]

Coverage increased (+0.3%) to 80.069% when pulling 466f911 on offtherailz:styles_wms into b956654 on geosolutions-it:master.