Theme Support

[vkbo]

Syntax colours are now saved in a config file, that together with the main style sheet, lives in the folder nw/themes/default. Other folders can be added with other sets of files, and the folder name used as the theme setting in the config file.

The folder must contain the files styles.css and theme.conf.

This PR also moves the editor and viewer into its own splitter, and sets the headers in the viewer to the same colour as in the editor.

[codecov-io]

Codecov Report

Merging #15 into master will increase coverage by 0.35%.
The diff coverage is 91.12%.

@@            Coverage Diff             @@
##           master      #15      +/-   ##
==========================================
+ Coverage   76.67%   77.03%   +0.35%     
==========================================
  Files          22       23       +1     
  Lines        2847     2939      +92     
==========================================
+ Hits         2183     2264      +81     
- Misses        664      675      +11

Impacted Files	Coverage Δ
nw/gui/docviewer.py	100% <100%> (ø)	⬆️
nw/gui/dochighlight.py	100% <100%> (ø)	⬆️
nw/gui/winmain.py	70% <100%> (ø)	⬆️
nw/config.py	95.28% <100%> (+0.3%)	⬆️
nw/gui/doceditor.py	64.94% <100%> (ø)	⬆️
nw/theme.py	85.52% <85.52%> (ø)

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