use Shift+Ctrl+- as shortcut key for divider
use Shift+Ctrl+- as shortcut key for divider

The previous shortcut, Ctrl+d on Windows and *nix, Cmd+d on macOS, collides with the "delete forward" shortcut for browsers.

Shift+Ctrl+- is the [Horizontal Rule shortcut for Evernote](https://help.evernote.com/hc/en-us/articles/209004807-What-are-the-keyboard-shortcuts-in-Evernote-for-Windows-), so I think it makes sense here.
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