Make MarkdownViewer used in HelpScreen work like a browser (back/forward)

Requires:

• !148 (merged) (Cause there are fixes from the textual side for links used in MarkdownViewer)

I had to use markdown_viewer.go method (because only then the stack is created). Otherwise, while we pass the text like we did before via the MarkdownViewer initializer, we won't be able to have navigation working properly (action back, action forward)

This MR:

• Links in help markdown works (navigate to other section in current file / other file)
• New actions added to the help viewer
  
• New screen when no context help is found