add scrollbar and fine-grained control to jump to unread messages in conversation list

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

...roject/briar/desktop/conversation/ConversationItemView.kt

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

...briar/desktop/conversation/ConversationMessageItemView.kt

.../briar/desktop/conversation/ConversationNoticeItemView.kt

...briar/desktop/conversation/ConversationRequestItemView.kt

...rproject/briar/desktop/conversation/ConversationScreen.kt

...oject/briar/desktop/conversation/ConversationViewModel.kt

...main/kotlin/org/briarproject/briar/desktop/theme/Icons.kt

...tlin/org/briarproject/briar/desktop/utils/PreviewUtils.kt

src/main/resources/strings/BriarDesktop.properties