- Add right-click context menu with undo/redo/cut/copy/paste/select all options, as well as Delete block and Move block to another buffer
- Add Delete Block and Move Block to another buffer in the application menu
- Change so that we use our custom "undo" event instead of relying on the default undo event, since there were cases when the default undo didn't trigger the CodeMirror undo reliably
- The Ctrl/Cmd-S keyboard shortcut now pops up a buffer search dialog that can be used to select a buffer that the block should be moved to.
- Update `BufferSelector` UI and logic to support moving blocks
