More Gui Tweaks

[vkbo]

This PR:

• Adds the option to show tabs/spaces and line endings in the editor. Suggestion Show tabs, spaces and line endings in editor #88.
• Cleans up the Preferences GUI a bit.

[codecov-io]

Codecov Report

Merging #90 into master will decrease coverage by 0.15%.
The diff coverage is 32.46%.

@@            Coverage Diff             @@
##           master      #90      +/-   ##
==========================================
- Coverage   63.04%   62.88%   -0.16%     
==========================================
  Files          42       42              
  Lines        6575     6601      +26     
==========================================
+ Hits         4145     4151       +6     
- Misses       2430     2450      +20

Impacted Files	Coverage Δ
nw/gui/mainmenu.py	93.4% <0%> (ø)	⬆️
nw/gui/configeditor.py	5.48% <0%> (-0.25%)	⬇️
nw/gui/doctree.py	50.26% <100%> (ø)	⬆️
nw/config.py	92.64% <100%> (+0.15%)	⬆️
nw/gui/doceditor.py	55.52% <50%> (-0.28%)	⬇️

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