started labelling window control buttons with more descriptive acessibility descriptions

bigbluebutton-client/locale/en_US/bbbResources.properties

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

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

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

...igbluebutton/modules/listeners/views/ListenersWindow.mxml

...uebutton/modules/present/ui/views/PresentationWindow.mxml

...org/bigbluebutton/modules/present/ui/views/SlideView.mxml

...rg/bigbluebutton/modules/viewers/views/ViewersWindow.mxml