Icon cleanup

[vkbo]

This PR removes icons from the main menu and context menu, and adds a set of simple icons for the dark theme. The systems theme will still pull the default OS icons.

[codecov-io]

Codecov Report

Merging #53 into master will increase coverage by <.01%.
The diff coverage is 87.23%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
+ Coverage   74.46%   74.47%   +<.01%     
==========================================
  Files          30       30              
  Lines        4934     4955      +21     
==========================================
+ Hits         3674     3690      +16     
- Misses       1260     1265       +5

Impacted Files	Coverage Δ
nw/gui/doceditor.py	62.03% <0%> (+0.16%)	⬆️
nw/gui/configeditor.py	5.73% <0%> (-0.02%)	⬇️
nw/gui/searchbar.py	74.11% <100%> (+0.3%)	⬆️
nw/gui/mainmenu.py	93.61% <100%> (ø)	⬆️
nw/gui/projecteditor.py	93.61% <100%> (+0.01%)	⬆️
nw/gui/doctree.py	51.37% <66.66%> (+0.13%)	⬆️
nw/theme.py	61.76% <75%> (+0.85%)	⬆️

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