mdview: download ældre versioner

Note: The file mdview-4.0.2-arm64.dmg was marked as malicious on VirusTotal by "Microsoft" as "Trojan:Script/Wacatac.C!ml".
Document icon (issue #83)
There is now a dedicated icon for Markdown documents.
On Windows, if installed via the setup EXE file, the document icon is associated instead of the application icon. With MSI, the application icon is still associated.
Improved dialog styling (issue #81)
The borders of the dialogs are now unobtrusive.
Bugfixes
- Double-clicking a file in macOS Finder now actually opens that file, instead of the README (issue #76)
- Mermaid syntax was not escaped for HTML, causing problems with statements containing < and > (issue #78)
- Fixed handling of the escape key in the search dialog. It didn't close the dialog like the others
- Fixed handling of paths containing special characters, e.g. 证据.md could not be linked to (issue #85)

Documentation in installation folder
The installation folder contains now some additional files, that were hidden inside the application data archive file:
- README.md
- CONTRIBUTING.md
- etc.
Dark scrollbars
If the theme is set to dark mode, the scrollbars are now displayed dark, instead of light grey.
See issue #71.
Bugfixes
- Fixed brief flashing of the content blocking banner during application start
- Regression fix: it was not possible anymore to navigate to headers with non ASCII characters in them. See issue #70.
- Bugfix at file type handling: the checkbox in the settings dialog, deciding whether the file shall be rendered as Markdown, was unchecked, if the file handling contains big letters, e.g. .MD, causing the application to lose the association for rendering it as Markdown. See issue #72.

## Unblocked content can now be remembered (issue #61)

It is now possible, to unblock images permanently, meaning that media doesn't need to be unblocked
again after restarting the application.

## Single window position, independent of the document (issue #67)

The application settings have now an option for remembering the window position:

- "Remember for each document" (default, as previous versions): Remembers the window position for
each document
- "Remember single position": The application opens the window always at the last previously opened
position, independently of the opened document

## Bugfixes

### Issue #62

HTML anchors can now be defined also via name, additionally to the already supported id attribute.

- --app-path: Outputs the path to the installation directory and exits immediately
- --data-path: Outputs the path to the data directory (where besides other thinks, the settings are stored) and exist immediately
Other
- Fixed a regression at the context menu that too many entries appeared

- Minus and plus keys on numpad
- Control + mouse wheel

Bugfixes
- Mermaid diagrams are now displayed with readable colors in dark mode (issue #39)