created a new class CustomMdiWindow to proper handle the context menu

bigbluebutton-client/src/SharedNotesModule.mxml

...on-client/src/org/bigbluebutton/common/CustomMdiWindow.as

...on-client/src/org/bigbluebutton/main/views/LogWindow.mxml

...rc/org/bigbluebutton/main/views/MainApplicationShell.mxml

.../src/org/bigbluebutton/main/views/NetworkStatsWindow.mxml

.../src/org/bigbluebutton/main/views/ShortcutHelpWindow.mxml

...igbluebutton/modules/broadcast/views/BroadcastWindow.mxml

...nt/src/org/bigbluebutton/modules/chat/views/ChatView.mxml

.../src/org/bigbluebutton/modules/chat/views/ChatWindow.mxml

...dules/deskshare/view/components/DesktopPublishWindow.mxml

.../modules/deskshare/view/components/DesktopViewWindow.mxml

.../org/bigbluebutton/modules/example/ExampleChatWindow.mxml

.../org/bigbluebutton/modules/layout/views/LayoutsCombo.mxml

...rc/org/bigbluebutton/modules/notes/views/NotesWindow.mxml

...bigbluebutton/modules/polling/views/CreatePollWindow.mxml

...bluebutton/modules/polling/views/DisplayResultWindow.mxml

...bigbluebutton/modules/polling/views/PollCreateWindow.mxml

...g/bigbluebutton/modules/polling/views/PollMainWindow.mxml

...g/bigbluebutton/modules/polling/views/TakePollWindow.mxml

...bigbluebutton/modules/polling/views/UpdatePollWindow.mxml