Support attachments to internal note file.
- Add/Delete/Clear/Sort;
- Support custom attachment folder for each notebook (read-only);
- Support renaming attachment;
1. Bug fix: after deleting last folder, the file list won't be cleared;
2. Move cursor one line down after inserting title automatically;
3. Prompt user to create a root folder after creating a notebook;
4. Autocomplete the folder name from the chosen path when creating a
notebook;
5. Add "Open Notebook Location" action in the context menu of
VNotebookSelector;
In Captain Mode, W will trigger the Navigation Mode. In this mode,
registered widgets will display some characters for hints and after user
pressing the the keys, corresponding widget will take action as response.
For now, only VNotebookSelector registers in the Navigation Mode.
