updates to overlay: add service to manage number of open overlays, add backdrop for overlay + better support for center overlay

src/Umbraco.Web.UI.Client/src/less/components/editor.less

@@ -4,7 +4,7 @@
 
 .umb-editor-wrapper{
 	background: white;
-	position: absolute; 
+	position: absolute;
 	top: 0px; bottom: 0px; left: 0px; right: 0px;
 }
 
@@ -19,7 +19,7 @@
 	left: 0px;
 }
 
-	
+
 .umb-editor-container{
 	top: 101px;
 	left: 0px;
@@ -30,6 +30,9 @@
 	overflow: auto;
 }
 
+.umb-editor-container.-stop-scrolling {
+	overflow: hidden;
+}
 
 .umb-editor-drawer{
 	margin: 0;