Hide cursor on next-unread. Fix #431.

NetNewsWire/MainWindow/MainWindowController.swift

@@ -203,6 +203,8 @@ class MainWindowController : NSWindowController, NSUserInterfaceValidations {
 			return
 		}
 
+		NSCursor.setHiddenUntilMouseMoves(true)
+		
 		if timelineViewController.canGoToNextUnread() {
 			goToNextUnreadInTimeline()
 		}

NetNewsWire/MainWindow/Sidebar/SidebarViewController.swift

@@ -192,6 +192,7 @@ import RSCore
 			return
 		}
 		
+		NSCursor.setHiddenUntilMouseMoves(true)
 		outlineView.selectRowIndexes(IndexSet([row]), byExtendingSelection: false)
 	}
 

NetNewsWire/MainWindow/Timeline/TimelineViewController.swift

@@ -280,6 +280,7 @@ class TimelineViewController: NSViewController, UndoableCommandRunner {
 		guard let ix = indexOfNextUnreadArticle() else {
 			return
 		}
+		NSCursor.setHiddenUntilMouseMoves(true)
 		tableView.rs_selectRow(ix)
 		tableView.scrollTo(row: ix)
 	}