add MessageCounter to count unread messages per contact and on startup

...in/kotlin/org/briarproject/briar/desktop/DesktopModule.kt

...lin/org/briarproject/briar/desktop/contact/ContactCard.kt

...iarproject/briar/desktop/conversation/ConversationList.kt

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

...sktop/notification/linux/LibnotifyNotificationProvider.kt

...rg/briarproject/briar/desktop/threading/BriarExecutors.kt

...riarproject/briar/desktop/threading/BriarExecutorsImpl.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

...n/kotlin/org/briarproject/briar/desktop/ui/NumberBadge.kt

...n/org/briarproject/briar/desktop/viewmodel/DbViewModel.kt

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

...otlin/org/briarproject/briar/desktop/DesktopTestModule.kt

...iar/desktop/notification/linux/TestNativeNotifications.kt