Corners patch 2

[bynect]

This is the 2* part of #1227.

Before we added the scaffolding for custom corners and fixed their behavior.
With this patch we add the ability to customize which corners to round trough the options:
corners, progress_bar_corners and icon_corners

Along with these options I updated the documentation, dunstrc and added a couple tests.

[bynect]

it seems like the string parsing causes problem with valgrind...

[codecov-commenter]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (ed5bcb4) 64.53% compared to head (275ed1b) 64.86%.

Files	Patch %	Lines
src/draw.c	0.00%	5 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1268      +/-   ##
==========================================
+ Coverage   64.53%   64.86%   +0.32%     
==========================================
  Files          48       48              
  Lines        7914     7954      +40     
==========================================
+ Hits         5107     5159      +52     
+ Misses       2807     2795      -12     

Flag	Coverage Δ
unittests	64.86% <89.36%> (+0.32%)	⬆️

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[bynect]

@fwsmit ready for me 👍🏻