SH-3909 Support avatar height offset

Adding a new visual param that allows users to manually adjust an offset for
how far off the ground (+ or -) their avatar's root bone is.

Supports the +-2m range people are used to adjusting in their viewers, but
new implementation should support server-generated appearances.

Conflicts:

	indra/newview/character/avatar_lad.xml
	indra/newview/llagent.cpp
	indra/newview/llcallingcard.cpp
	indra/newview/skins/default/xui/en/strings.xml

indra/newview/character/avatar_lad.xml

@@ -823,6 +823,23 @@
        		<bone name="mNeck" scale="0 0 .5" />
          </param_skeleton>
       </param>
+
+    <param
+     id="11001"
+     group="0"
+     name="Hover"
+     wearable="shape"
+     edit_group="shape_body"
+     edit_group_order="4"
+     label_min="Lower"
+     label_max="Higher"
+     value_min="-2"
+     value_max="2"
+     value_default="0"
+     camera_distance="2.5">
+      <param_skeleton />
+    </param>
+
    </skeleton>
 
    <mesh