DRAFT: fix Markdown link click behaviour

[lowekoz]

Running examples\markdown.py and clicking the example.md from the demo app will also open the relative file link ./example.md in the browser, which is a bit wonky behaviour. Relates to #5171 #5169, suggested fix by separating behaviour of external link protocols (http://,https://,ftp://...) and local resource path/anchor. With fix links won't trigger the browser open_url command, only external links will (if open_links=True), and external links won't trigger local navigation changes.

On approval TODOs:

• Docstrings on all new or modified functions / classes
• Updated documentation(?)
• Updated CHANGELOG.md (where appropriate)
• Tests (?)