Hover over panel and Cmd+D to split

[amacneil]

Description
I want to be able to easily split panels using keyboard shortcuts.

As a mac/iterm user, Cmd+d for for a vertical split and Cmd+Shift+d for a horizontal split would make the most sense. We should research common shortcuts on windows/linux.

I propose that we combine this split behavior with "focus follows mouse", so those keyboard shortcuts will split whichever pane my mouse is currently over.

cc @2metres @jtbandes @defunctzombie for thoughts

[rgov]

Note that ⌘⇧D on macOS Terminal.app closes a (vertical) pane, it doesn't create a new one. (It is a feature I never use so I'm not personally offended if there's a conflict here.) I couldn't find a key combo for this in Xcode. Visual Studio Code seems to default to ⌘\.