Use Qt's internal markdown formatting

Qt gained the ability to understand Markdown some while back. We now switched to using this internal version over using ShowdownJS (which seems to have some long lasting errors that won't get fixed, such as wrong list indentation).

In addition, we also use a read only TextEditor to show the notes of an item. This fixes another rendering issue of Qt (and allows users to select parts of the text and copy it view a new context menu).

Closes #575 (closed)