Detach preview from editor

First of all, awesome work on this, cheers!

I have a dual monitor setup, one of which is in portrait mode (great for reading documents). It would be amazing if I could detach the preview window and move this onto my portrait monitor and make use of it to get a better overview.

Thanks!

Start by locating the editor's preview-window implementation and reviewing how MacDown currently manages its windows. Determine the window behavior needed for the preview to be detached and moved to another monitor, then verify that the preview can operate independently from the editor.

Written by the indexing model from the issue text.