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]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

❌ Patch coverage is 89.36170% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.79%. Comparing base (b6a8a19) to head (accf34f).
⚠️ Report is 356 commits behind head on master.

Files with missing lines	Patch %	Lines
src/draw.c	0.00%	5 Missing ⚠️
❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1268      +/-   ##
==========================================
+ Coverage   64.46%   64.79%   +0.32%     
==========================================
  Files          48       48              
  Lines        7920     7960      +40     
==========================================
+ Hits         5106     5158      +52     
+ Misses       2814     2802      -12     

Flag	Coverage Δ
unittests	64.79% <89.36%> (+0.32%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[bynect]

@fwsmit ready for me 👍🏻