Add Missing Includes Across src/libslic3r

Every libslic3r source and header now directly includes the headers declaring what it uses, rather than relying on the precompiled header or transitive includes. Generated with clang-tidy misc-include-cleaner, with libslic3r headers spelled libslic3r/... so they resolve outside the library's private include paths. MultiMaterialSegmentation.hpp, Support/SupportParameters.hpp and Format/STEP.hpp are made self-contained by hand.

src/libslic3r/ProjectTask.hpp

@@ -1,6 +1,7 @@
 #ifndef slic3r_ProjectTask_hpp_
 #define slic3r_ProjectTask_hpp_
 
+#include <functional>
 #include <map>
 #include <vector>
 #include <string>