Add tooltip to explain why Advanced Edit button is disabled. b=161113. r=brade, sr=alecf

editor/ui/composer/locale/en-US/editor.properties

@@ -165,6 +165,7 @@ MissingPublishSiteError=No publishing site information given. Switching to Setti
 #LOCALIZATION NOTE (DuplicateSiteNameError): Don't translate %name%
 DuplicateSiteNameError="%name%" already exists. Please enter a different site name.
 AdvancedProperties=Advanced Properties...
+AdvancedEditForCellMsg=Advanced Edit is unavailable when multiple cells are selected
 # LOCALIZATION NOTE (ObjectProperties):Don't translate "%obj%" it will be replaced with one of above object nouns
 ObjectProperties=%obj% Properties...
 # LOCALIZATION NOTE This character must be in the above string and not confict with other accesskeys in Format menu

editor/ui/dialogs/content/EdTableProps.js

@@ -718,7 +718,11 @@ function DoCellSelection()
 
   // Currently, we can only allow advanced editing on ONE cell element at a time
   //   else we ignore CSS, JS, and HTML attributes not already in dialog
-  SetElementEnabledById("AdvancedEditButton2", gSelectedCellCount == 1);
+  SetElementEnabled(gDialog.AdvancedEditCell, gSelectedCellCount == 1);
+
+  gDialog.AdvancedEditCell.setAttribute("tooltiptext", 
+    gSelectedCellCount > 1 ? GetString("AdvancedEditForCellMsg") :
+                             gDialog.AdvancedEditCellToolTipText);
 }
 
 function SetSelectionButtons()