* 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
1. In small overhang detection, some sharp tail cases are missed because
the max_layer was not compared.
2. In normal support, previously the lower layer's expolys were not right,
should only keep the ones that won't vanish when extruding.
Jira: STUDIO-1977, STUDIO-2200
Change-Id: Ia09ff1ab4870b7e3c199a827932536867644c76c
(cherry picked from commit e3ba3a3114821e951d6eb00b4c95e6e63ffe00d4)
