Implement org.freedesktop.appearance.color-scheme support on Linux

[vimpostor]

This finally allows us to follow the system theme without restart in a reliable and desktop-environment independent way.

This uses the new standardized org.freedesktop.appearance.color-scheme key to accomplish this.

The following DEs already support this in the respective version:

• KDE Plasma since version 5.24 (Released 2022-02-08)
• Gnome since version 42 (Unreleased as of now)

In addition to that, the xdg-deskop-portal user service needs to be running.

Relevant blog post: https://blogs.gnome.org/alexm/2021/10/04/dark-style-preference/

Properly fixes #7146 (was closed as it was basically impossible to implement this in a reliable way a few years back)

Note that if the user does not have xdg-desktop-portal or is missing support for that specific key, the previous behaviour is unchanged.

Short demo:

demo.mp4

Type of change

• ✅ New feature (change that adds functionality)

[droidmonkey]

Awesome!

[codecov-commenter]

Codecov Report

Merging #7422 (19f08c3) into develop (58615d7) will increase coverage by 0.01%.
The diff coverage is 70.59%.

@@             Coverage Diff             @@
##           develop    #7422      +/-   ##
===========================================
+ Coverage    64.33%   64.33%   +0.01%     
===========================================
  Files          339      339              
  Lines        43199    43216      +17     
===========================================
+ Hits         27788    27802      +14     
- Misses       15411    15414       +3     

Impacted Files	Coverage Δ
src/gui/osutils/nixutils/NixUtils.h	50.00% <ø> (ø)
src/gui/osutils/nixutils/NixUtils.cpp	33.53% <70.59%> (+4.12%)	⬆️
src/core/Entry.cpp	82.76% <0.00%> (+0.20%)	⬆️

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[vimpostor]

FYI I think Gnome might be violating their own spec, i.e. they return "No preference" instead of "Prefers light theme" when the user explicitly chooses a light theme. I am not completely sure about this, and since Gnome 42 hasn't been released yet, it can still change, but it is something to keep in mind.
Feel free to ping me, if such an issue ever comes up.

The problem would be easy to fix (just interpet "No preference" as light preference), however I think Gnome should just report the correct preference ("prefers light theme") in the first place instead of shipping such a broken Portal implementation.

[droidmonkey]

Have you filed a bug report with them?

[vimpostor]

Have you filed a bug report with them?

Well not yet, as I am not sure if my assumption is even true. I only assume this, because their blog post was hinting towards this, I yet have to await the Gnome release to see if it's actually the case.