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.

Start by locating the new toolbar implementation and its button-state logic. Trace how selection and undo/redo history are represented, then verify that Cut and Copy reflect selection state and Undo and Redo reflect available history; done means irrelevant buttons are disabled.

Written by the indexing model from the issue text.