More sofisticated control of sharednotes undo button

...src/main/scala/org/bigbluebutton/core/api/Constants.scala

...c/main/scala/org/bigbluebutton/core/api/OutMessages.scala

...in/scala/org/bigbluebutton/core/apps/SharedNotesApp.scala

.../scala/org/bigbluebutton/core/apps/SharedNotesModel.scala

...re/pubsub/senders/SharedNotesMessageToJsonConverter.scala

...ain/java/org/bigbluebutton/common/messages/Constants.java

...bluebutton/common/messages/PatchDocumentReplyMessage.java

...luebutton/red5/client/SharedNotesClientMessageSender.java

...uebutton-client/branding/default/style/css/BBBDefault.css

...default/style/css/assets/images/arrows_redo_icon_grey.png

...default/style/css/assets/images/arrows_undo_icon_grey.png

...luebutton/modules/sharednotes/events/ReceivePatchEvent.as

...luebutton/modules/sharednotes/services/MessageReceiver.as

...uebutton/modules/sharednotes/views/SharedNotesWindow.mxml