v2.0.0-alpha.4

• Support multiple windows so that multiple documents can be previewed at the same time. Now a single Shiba process can open multiple windows. When the last window is closed, or 'Quit' menu item or key action is selected, the process exits.
  • Implement the following ways to open new windows in several situations.
    • Clicking a link to local markdown file with pressing Shift key opens the file in a new window.
    • Clicking a hash link (e.g. [link](#hash)) with pressing Shift key opens the current document in a new window and automatically scrolling to the linked element.
    • 'New Window' menu item or NewWindow key action opens a new empty window.
    • 'Duplicate Window' menu item or DuplicateWindow key action duplicates the current window.
      • This key action is assigned to Ctrl+N by default.
    • 'Open in New Window' menu item or OpenFileInNewWindow key action picks files in a file dialog and opens them in respective new windows.
    • In the history palette, hit Enter or click a history item with pressing Shift to open the selected item in a new window.
    • In the outline palette, hit Enter or click a section with pressing Shift to open the current document in a new window and automatically scroll to the selected section.
    • Click a section in the side bar to open the current document in a new window and automatically scroll to the selected section.
    • --open (or -o) option in the command line arguments opens a given path with a new window. See --help output for more details.
  • Add the following menu items and key actions related to multiple windows.
    • 'Close Window' menu item and CloseWindow key action to close the current window.
      • This key action is assigned to Ctrl+W by default.
    • 'Close All Other Windows' menu item and CloseAllOtherWindows key action to close all windows except the current one.
    • 'Bring all to front' window menu item on macOS.
• Return non-zero exit status when at least one unexpected error was caused.
• Fix notifications are included in a printed pages.
• Update wry to 0.55 and tao to 0.35.
• Don't set 'pre-release' on GitHub release page so that the latest alpha release is linked from the repository page.