Disable toolbar buttons when not relevant

The new toolbar is useful, but sometimes misleading. If there's no selection, then Cut and Copy should be disabled. If Undo or Redo are not possible (because there's no history in that direction), then they should be disabled, as well. In general, if there were an easy way to enable or disable toolbar buttons, then there are probably a lot of checks we could add to guard them.

首先定位新的工具栏实现及其按钮状态逻辑。跟踪选择状态和撤销/重做历史的表示方式，然后验证 Cut 和 Copy 是否反映选择状态，以及 Undo 和 Redo 是否反映可用历史；当无关按钮被禁用时即表示完成。

由索引模型根据 Issue 内容生成。