DEV-44367: Put back help IDs for prefs, land, and water.

The help_topic tags for Preferences, About Land, and Advanced Water
Editor were removed for EXT-2766. However, this has the sideeffect
that those floaters no longer show the "?" Help button. So, I'm
reverting that change and putting these help IDs back again.

indra/newview/skins/default/xui/en/floater_about_land.xml

@@ -4,6 +4,7 @@
  height="420"
  layout="topleft"
  name="floaterland"
+ help_topic="floaterland"
  save_rect="true"
  title="ABOUT LAND"
  width="490">

indra/newview/skins/default/xui/en/floater_preferences.xml

@@ -7,6 +7,7 @@
  height="460"
  layout="topleft"
  name="Preferences"
+ help_topic="preferences"
  single_instance="true"
  title="PREFERENCES"
  width="620">

indra/newview/skins/default/xui/en/floater_water.xml

@@ -4,6 +4,7 @@
  height="240"
  layout="topleft"
  name="Water Floater"
+ help_topic="water_floater"
  save_rect="true"
  title="ADVANCED WATER EDITOR"
  width="700">