EXT-2765: Added help IDs for tabs in the Places sidebar.

The issue was that the widgets for this panel are not built solely
from XUI. There's code in LLPanelPlaces::changeInventory() that
creates the actual My Inventory and Teleport History panels. Adding
help_topic strings to the relevant XUI files for the LLLandmarksPanel
and LLTeleportHistoryPanel classes fixes this issue.

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

@@ -6,6 +6,7 @@
    layout="topleft" 
    left="0"
    width="380"
+   help_topic="panel_landmarks"
    border="true"
    background_visible="true"
    bg_alpha_color="DkGray2"

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

 <?xml version="1.0" encoding="utf-8" standalone="yes" ?>
 <panel name="Teleport History" bottom="0" height="326" left="0" width="380"
+     help_topic="panel_teleport_history"
      border="true" follows="left|top|right|bottom">     
     <accordion
      follows="left|top|right|bottom"