lightning-item-iframe: use collapsed attribute to display/hide HTML editor

As advised by the "Rapid Application Development with Mozilla" book by
Nigel McFarlane.

Indeed, a tag sole any binding when we use attribute like "display:none"
because a XBL binding requires a frame (styleable area).

Fixes #28

Author: Trim

chrome/content/lightning-item-iframe.js

@@ -151,7 +151,7 @@ exchangeEventDialog.prototype = {
 				this._document.getElementById("item-description").hidden = true;
 
 				// Display our own HTML content editor
-				itemBodyEditor.hidden = false;
+				itemBodyEditor.setAttribute("collapsed", "false");;
 
 				itemBodyEditor.setAttribute("scrollbars","yes");
 			}
@@ -256,7 +256,7 @@ exchangeEventDialog.prototype = {
 		if (aCalendar.type !== "exchangecalendar") {
 			// Hidde HTML content editor
 			this._document.getElementById("item-description").hidden = false;
-			this._document.getElementById("exchWebService-body-editor").hidden = true;
+			this._document.getElementById("exchWebService-body-editor").setAttribute("collapsed", "true");
 		}
 	},
 

chrome/content/lightning-item-iframe.xul

@@ -142,7 +142,7 @@
                     flex="1"
                     insertafter="item-description"
                     disable-on-readonly="true"
-                    hidden="true"/>
+                    collapsed="true"/>
             </tabpanel>
 
         </tabpanels>