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/libslic3r/Color.hpp

@@ -1,6 +1,8 @@
 #ifndef slic3r_Color_hpp_
 #define slic3r_Color_hpp_
 
+#include <vector>
+#include <string>
 #include <array>
 #include <algorithm>