The drag & drop behaviour of the text editor itself is kept as-is
(moving text blocks with the mouse etc., and also inserting file names
if dropped into the text area) because we don't want to re-implement all
of this ourselves.
However, we display a guidance text if a user drags files over the text
area telling them where to drop them to open.

And some cosmetic fixes for the line numbering of the preview editor

Moreover, the license for the lines numbering has been clarified and is
now advertised in the About window

Modernize signal mapping in PreviewEdit (better compile time checks), re-add line number safety margin (sorry, my oversight).

(old style cast, and "no out-of-line virtual method definitions")

Use recommended call for text width. I looked this up a while ago for the Alternative width calculation. This way we stay happy with the default Qt 5.9 and don't require 5.11 just for this call.

(Ok I admit - I don't want to update Qt myself. But luckily it is the correct call.)

The fonts are now setup in the constructor. An unnecessary script has
been deleted.

And a minor visual tweak to the line numbering

(it is licensed under BSD instead of GPL)