help-overlay: Add missed shortcut keys information.
A few shortcut keys were missed and it could make users get confused that one of them exist or not.
Add following keys
- Ctrl+D for adding bookmark.
- Ctrl+I for inverting colors.
- D for toggling dual page.
- N for going to next page.
- P for going to previous page.
- F9 for toggling sidebar.
- F7 for toggling caret navigation.
This is not only #890 (closed) but also other keybindings.