Separate Notebook Node Explorer

• NotebookExplorer: support separate node explorer
  
• Theme: add user-provided VSCode-Dark theme
  
• MarkdownEditor: use web to highlight code blocks
• MarkdownViewWindow
  • Add switch for code block line number
  • Fix ParseToMarkdown <style> issue
  • Add config for overridding MathJax script
• SortDialog: fix sorting issue of date
• FramelessMainWindow: fix StayOnTop issue