Documentation in installation folder

The installation folder contains now some additional files, that were hidden inside the application data archive file:

Dark scrollbars

If the theme is set to dark mode, the scrollbars are now displayed dark, instead of light grey.

See issue #71.

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.