Fixes #1788: configurability of SettingModal showElevationChart

[mbarto]

To configure it in the TOC plugin:

{
                 "name": "TOC",
                 "cfg": {
                  "settingsOptions":  {
                      "showElevationChart": false
                    }
                 }
}

[coveralls]

Coverage remained the same at 78.893% when pulling 3d19843 on mbarto:elevations_show_chart into b7479ce on geosolutions-it:master.