Make the sidebar’s outline view first responder on startup. Fixes #1262.

Mac/MainWindow/Sidebar/SidebarViewController.swift

@@ -75,6 +75,10 @@ protocol SidebarDelegate: class {
 			}
 			row += 1
 		}
+
+		DispatchQueue.main.async {
+			self.outlineView.window?.makeFirstResponder(self.outlineView)
+		}
 	}
 
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