A *lot* of import fixes for flatpak (#3909)

* A *lot* of import fixes for flatpak

Aside, thank you @hadess for the majority of these fixes. You are the
base point for a lot of issues fixed during the creation of this
flatpak.

* Use slic3r::load_string_file

Boost 1.84 removed `boost::fs::load_string_file` so use the func located
in Utils.hpp

src/slic3r/Utils/ColorSpaceConvert.cpp

@@ -1,6 +1,7 @@
 #include "ColorSpaceConvert.hpp"
 
 #include <algorithm>
+#include <cmath>
 
 const static float param_13 = 1.0f / 3.0f;
 const static float param_16116 = 16.0f / 116.0f;