private groups: show notifications and update unread message badges

...rg/briarproject/briar/desktop/forum/ForumListViewModel.kt

.../desktop/forum/conversation/ForumConversationViewModel.kt

...org/briarproject/briar/desktop/navigation/BriarSidebar.kt

...briarproject/briar/desktop/navigation/SidebarViewModel.kt

...riar/desktop/notification/AbstractNotificationProvider.kt

...roject/briar/desktop/notification/NotificationProvider.kt

...t/briar/desktop/notification/SoundNotificationProvider.kt

...iarproject/briar/desktop/privategroup/PrivateGroupItem.kt

...t/briar/desktop/privategroup/PrivateGroupListViewModel.kt

...tegroup/conversation/PrivateGroupConversationViewModel.kt

...ar/desktop/threadedgroup/ThreadedGroupMessageReadEvent.kt

...readedgroup/conversation/ThreadedConversationViewModel.kt

.../main/kotlin/org/briarproject/briar/desktop/ui/BriarUi.kt

...otlin/org/briarproject/briar/desktop/ui/MessageCounter.kt

...n/org/briarproject/briar/desktop/ui/MessageCounterImpl.kt

...esktop/src/main/resources/strings/BriarDesktop.properties