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 :warning:
src/option_parser.c	73.68%	5 Missing :warning:
src/rules.c	72.22%	5 Missing :warning:
src/draw.c	88.57%	4 Missing :warning:
src/notification.c	62.50%	3 Missing :warning:
src/dunst.c	0.00%	2 Missing :warning:
test/setting.c	91.66%	1 Missing :warning:

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Additional details and impacted files

```diff @@ 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](https://app.codecov.io/gh/dunst-project/dunst/pull/1370/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dunst-project) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/dunst-project/dunst/pull/1370/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dunst-project) | `65.51% <72.67%> (+0.14%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dunst-project#carryforward-flags-in-the-pull-request-comment) to find out more.

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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 🤣