Jump to first unread message with Shift+PgUp

This feature doesn't work very well at the moment, but that's not an
excuse not to have a keyboard shortcut for it. As usual, it's the same
shortcut as many other chat clients use.

src/qml/Page/RoomPage.qml

@@ -357,6 +357,7 @@ Kirigami.ScrollablePage {
                     messageListView.goToEvent(currentRoom.readMarkerEventId)
                 }
                 icon.name: "go-up"
+                shortcut: "Shift+PgUp"
             }
 
             QQC2.ToolTip {