Add color gradients

[bynect]

Supersedes #1286

[bynect]

I added the gradient only for the highlight at the moment, but it should be easy to add to the others. the cairo pattern is cached at the start. for the rest it is basically the same as #1286 but with a custom gradient type instead of a string list

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 72.67442% with 47 lines in your changes missing coverage. Please review.

Project coverage is 65.51%. Comparing base (c1cc6d1) to head (8630daf).

Files with missing lines	Patch %	Lines
src/dbus.c	15.62%	27 Missing ⚠️
src/option_parser.c	73.68%	5 Missing ⚠️
src/rules.c	72.22%	5 Missing ⚠️
src/draw.c	88.57%	4 Missing ⚠️
src/notification.c	62.50%	3 Missing ⚠️
src/dunst.c	0.00%	2 Missing ⚠️
test/setting.c	91.66%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1370      +/-   ##
==========================================
+ Coverage   65.37%   65.51%   +0.14%     
==========================================
  Files          50       50              
  Lines        8321     8468     +147     
  Branches      999     1021      +22     
==========================================
+ Hits         5440     5548     +108     
- Misses       2881     2920      +39     

Flag	Coverage Δ
unittests	65.51% <72.67%> (+0.14%)	⬆️

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[bynect]

this is what it looks like

[bynect]

@zappolowski @fwsmit should be ready. I already closed the old pr

[bynect]

ok fixed 🤣

[bynect]

@zappolowski @fwsmit can I merge this?