Implement switching between tabs with shortcut key

[aesilevich]

1. We need a way to switch between tabs using a shortcut key. We can use "CMD+}" / "CMD+{" as in Xcode or "CTRL+TAB" / "CTRL+SHIFT+TAB" as in many other applications.
2. We also probably need to add navigation menu items: "Show Previous Tab" / "Show Next Tab"