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/GUI/Widgets/RoundedRectangle.cpp

@@ -1,6 +1,7 @@
 #include "RoundedRectangle.hpp"
 #include "../wxExtensions.hpp"
 #include <wx/dcgraph.h>
+#include <wx/dcclient.h>
 
 BEGIN_EVENT_TABLE(RoundedRectangle, wxPanel)
 EVT_PAINT(RoundedRectangle::OnPaint)
@@ -32,4 +33,4 @@ void RoundedRectangle::OnPaint(wxPaintEvent &evt)
         dc.SetBrush(wxBrush(*wxTRANSPARENT_BRUSH));
         dc.DrawRoundedRectangle(0, 0, GetSize().GetWidth(), GetSize().GetHeight(), m_radius);
     }
-}
+}