Add "Open directory as collection" menu item accelerator key

[IonoclastBrigham]

Unlike the other "open" items under the File menu, "open directory" has no keyboard shortcut associated with it. I propose Ctrl+D / ⌘+D, depending on platform.

I already have a working implementation, and can create a PR if there's no further input or discussion.

[lehitoskin]

Sounds good to me.