Suggestion: Browse mode (open folder)

[ppo]

Use case: Writing/testing the doc of a Git project.

Browse mode:

• View: navigation sidebar | code | preview
• Behavior: preview (single file) or code (multiple files)

Features:

• Open a folder (i.e. docs/) ⇒ activates the Browse mode
• Sidebar shows the folder structure with the files
• Toggle view all files / .md only
• Toggle single-file / each file in a separate tab
• Clicking a link to a local file in a page opens that file in MacDown
• Preview also images

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Somewhat similar to #1336